As filed with the Securities and Exchange Commission on September 22, 2017.

Registration No. 333-     

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

FORM S-4
REGISTRATION STATEMENT

UNDER
THE SECURITIES ACT OF 1933

FIRST FINANCIAL BANCORP.
(Exact Name of Registrant as Specified in its Charter)

255 East Fifth Street, Suite 700
Cincinnati, Ohio 45202
(877) 322-9530
(Address, including Zip Code, and Telephone Number, including Area Code, of Registrant’s Principal Executive Offices)

Shannon M. Kuhl
Senior Vice President and Chief Legal Officer
255 East Fifth Street, Suite 700
Cincinnati, Ohio 45202
(513) 979-5773
(Name, Address, including Zip Code, and Telephone Number, including Area Code, of Agent for Service)

With copies to:

Approximate date of commencement of the proposed sale of the securities to the public:

As soon as practicable after this Registration Statement becomes effective and upon completion of the merger described in the enclosed document.

If the securities being registered on this Form are being offered in connection with the formation of a holding company and there is compliance with General Instruction G, check the following box. o

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act of 1933, as amended, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. o

If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. o

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company” and “emerging growth company” in Rule 12b-2 of the Exchange Act. (Check one):

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act.   ☐

If applicable, place an X in the box to designate the appropriate rule provision relied upon in conducting this transaction:

Exchange Act Rule 13e-4(i) (Cross-Border Issuer Tender Offer) o

Exchange Act Rule 14d-1(d) (Cross-Border Third-Party Tender Offer) o

CALCULATION OF REGISTRATION FEE

The Registrant hereby amends this Registration Statement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment which specifically states that this Registration Statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act, or until the Registration Statement shall become effective on such date as the Commission, acting pursuant to said Section 8(a), may determine.

PRELIMINARY—SUBJECT TO COMPLETION—SEPTEMBER 22, 2017

MERGER PROPOSED—YOUR VOTE IS VERY IMPORTANT

Dear Shareholder:

On July 25, 2017, First Financial Bancorp., or First Financial, and MainSource Financial Group, Inc., or MainSource, entered into an Agreement and Plan of Merger (which we refer to as the “merger agreement”) that provides for the combination of the two companies. Under the merger agreement, MainSource will merge with and into First Financial, with First Financial as the surviving corporation, in a transaction we refer to as the “merger.” Following the completion of the merger, MainSource Bank, a wholly-owned bank subsidiary of MainSource, will merge with and into First Financial Bank, a wholly-owned subsidiary of First Financial, with First Financial Bank as the surviving bank, in a transaction we refer to as the “bank merger.” The merger will combine two Midwest community banks to create a $13 billion institution with scale and strength in commercial and retail banking, wealth management, and specialty finance.

In the merger, each outstanding share of MainSource common stock (except for certain specified shares of MainSource common stock held by MainSource or First Financial) will be automatically converted into the right to receive 1.3875 shares of First Financial common stock (which we refer to as the “exchange ratio”). Although the number of shares of First Financial common stock that each MainSource shareholder will receive is fixed, the market value of the merger consideration will fluctuate with the market price of First Financial common stock and will not be known at the time MainSource shareholders vote on the merger. Based on the closing price of First Financial’s common stock on The NASDAQ Stock Market LLC, or NASDAQ, on July 25, 2017, the last trading day before public announcement of the merger, the exchange ratio represented approximately $38.99 in value for each share of MainSource common stock. Based on First Financial’s closing price on [   ], 2017, of $[ ], the exchange ratio represented approximately $[   ] in value for each share of MainSource common stock. Based on the exchange ratio and the number of shares of MainSource common stock outstanding and reserved for issuance under various equity plans as of [       ], 2017, the maximum number of shares of First Financial common stock issuable in the merger is [       ]. We urge you to obtain current market quotations for First Financial (trading symbol “FFBC”) and MainSource (trading symbol “MFSG”).

MainSource and First Financial will each hold a special meeting of their shareholders in connection with the merger. MainSource and First Financial shareholders will be asked to vote to adopt the merger agreement and approve related matters, as described in the attached joint proxy statement/prospectus. Adoption of the merger agreement requires the affirmative vote of the holders of two-thirds of the outstanding shares of First Financial common stock and the affirmative vote of the holders of a majority of the outstanding shares of MainSource common stock.

The special meeting of First Financial shareholders will be held on [       ] at [       ] local time. The First Financial special meeting may be attended in person, at the Taft Centre at Fountain Square, 425 Walnut Street, Second Floor, Cincinnati, Ohio 45202, or viewed through a webcast via the Internet at www.virtualshareholdermeeting.com/ffbc17SM.

The special meeting of MainSource shareholders will be held on [       ] at 2105 North State Road 3 Bypass, Greensburg, Indiana 47240, at [       ] local time.

MainSource’s board of directors unanimously recommends that MainSource shareholders vote “FOR” the adoption of the merger agreement and “FOR” the other matters to be considered at the MainSource special meeting.

First Financial’s board of directors unanimously recommends that First Financial shareholders vote “FOR” the adoption of the merger agreement and “FOR” the other matters to be considered at the First Financial special meeting.

The attached joint proxy statement/prospectus describes the special meeting of First Financial, the special meeting of MainSource, the merger, the documents related to the merger, and other related matters. Please carefully read the entire joint proxy statement/prospectus, including “Risk Factors” beginning on page 25, for a discussion of the risks relating to the merger. You also can obtain information about First Financial and MainSource from documents that each has filed with the Securities and Exchange Commission.

Claude E. Davis Chief Executive Officer First Financial Bancorp.	Archie M. Brown, Jr. President and Chief Executive Officer MainSource Financial Group, Inc.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of the securities to be issued in the merger or passed upon the adequacy or accuracy of this joint proxy statement/prospectus. Any representation to the contrary is a criminal offense.

The securities to be issued in the merger are not savings or deposit accounts or other obligations of any bank or non-bank subsidiary of either First Financial or MainSource, and they are not insured by the Federal Deposit Insurance Corporation or any other governmental agency.

The date of this joint proxy statement/prospectus is [       ], 2017, and it is first being mailed or otherwise delivered to the shareholders of First Financial and MainSource on or about [       ], 2017.

NOTICE OF SPECIAL MEETING OF SHAREHOLDERS

To the Shareholders of First Financial Bancorp.:

First Financial Bancorp. (which we refer to as “First Financial”) will hold a special meeting (which we refer to as the “First Financial special meeting”) of holders of common stock of First Financial (which we refer to as “First Financial shareholders”) at [       ] local time, on [       ]. You may attend the First Financial special meeting in person at the Taft Centre at Fountain Square, 425 Walnut Street, Second Floor, Cincinnati, Ohio 45202. You may also view a webcast of the First Financial special meeting via the Internet at www.virtualshareholdermeeting.com/ffbc17SM when you enter your 12-digit control number included with the proxy card. Instructions on how to view the First Financial special meeting via the webcast are posted at www.virtualshareholdermeeting.com/ffbc17SM. You will not be able to vote your shares during the First Financial special meeting if you are attending via webcast. While our management will address questions from First Financial shareholders physically present or who have submitted their questions electronically prior to the First Financial special meeting, the webcast will not allow you to ask questions of management during the First Financial special meeting. You may visit www.proxyvote.com at any time prior to the First Financial special meeting to ask questions of our executive management that may be addressed in the First Financial special meeting. The purpose of the First Financial special meeting will be to consider and vote upon the following matters:

We have fixed the close of business on [       ] as the record date for the First Financial special meeting. Only First Financial shareholders of record at that time are entitled to notice of, and to vote at, the First Financial special meeting, or any adjournment or postponement of the First Financial special meeting. Approval of the First Financial merger proposal requires the affirmative vote of holders of two-thirds of the outstanding shares of common stock of First Financial (which we refer to as “First Financial common stock”). The First Financial adjournment proposal will be approved if a majority of the votes present and entitled to vote at the First Financial special meeting are voted in favor of the adjournment proposal.

First Financial’s board of directors has unanimously approved the merger agreement, has determined that the merger agreement and the transactions contemplated thereby, including the merger, are advisable and in the best interests of First Financial and its shareholders, and unanimously recommends that First Financial shareholders vote “FOR” the First Financial merger proposal and “FOR” the First Financial adjournment proposal.

Your vote is very important. We cannot complete the merger unless First Financial’s shareholders adopt the merger agreement.

Regardless of whether you plan to attend the First Financial special meeting, please vote as soon as possible. If you hold stock in your name as a shareholder of record of First Financial, please complete, sign, date, and return the accompanying proxy card in the enclosed postage-paid return envelope. If you hold your stock in “street name” through a bank or broker, please follow the instructions on the voting instruction card furnished by the record holder.

The enclosed joint proxy statement/prospectus provides a detailed description of the First Financial special meeting, the merger, the documents related to the merger, and other related matters. We urge you to read the joint proxy statement/prospectus, including any documents incorporated in the joint proxy statement/prospectus by reference, and its annexes carefully and in their entirety.

	BY ORDER OF THE BOARD OF DIRECTORS,
	Claude E. Davis Chief Executive Officer First Financial Bancorp.

NOTICE OF SPECIAL MEETING OF SHAREHOLDERS

To the Shareholders of MainSource Financial Group, Inc.:

MainSource Financial Group, Inc. (which we refer to as “MainSource”) will hold a special meeting of holders of common stock of MainSource (which we refer to as “MainSource shareholders”) at [         ] local time, on [         ], at 2105 North State Road 3 Bypass, Greensburg, Indiana 47240 (which we refer to as the “MainSource special meeting”) to consider and vote upon the following matters:

We have fixed the close of business on [       ] as the record date for the MainSource special meeting. Only MainSource shareholders of record at that time are entitled to notice of, and to vote at, the MainSource special meeting, or any adjournment or postponement of the MainSource special meeting. Approval of the MainSource merger proposal requires the affirmative vote of holders of a majority of the outstanding shares of common stock of MainSource (which we refer to as “MainSource common stock”). Approval of each of the MainSource compensation proposal and the MainSource adjournment proposal requires a majority of the votes present and entitled to vote at the MainSource special meeting to be voted in favor of such proposal.

MainSource’s board of directors has unanimously approved the merger agreement, has determined that the merger, on the terms and conditions set forth in the merger agreement, is advisable and in the best interests of MainSource and its shareholders, and unanimously recommends that MainSource shareholders vote “FOR” the MainSource merger proposal, “FOR” the MainSource compensation proposal and “FOR” the MainSource adjournment proposal.

Your vote is very important. We cannot complete the merger unless MainSource’s shareholders adopt the merger agreement.

Regardless of whether you plan to attend the MainSource special meeting, please vote as soon as possible. If you hold stock in your name as a shareholder of record of MainSource, please complete, sign, date, and return the accompanying proxy card in the enclosed postage-paid return envelope. If you hold your stock in “street name” through a bank or broker, please follow the instructions on the voting instruction card furnished by the record holder.

The enclosed joint proxy statement/prospectus provides a detailed description of the MainSource special meeting, the merger, the documents related to the merger, and other related matters. We urge you to read the joint proxy statement/prospectus, including any documents incorporated in the joint proxy statement/prospectus by reference, and its annexes carefully and in their entirety.

	BY ORDER OF THE BOARD OF DIRECTORS,
	Archie M. Brown, Jr. President and Chief Executive Officer MainSource Financial Group, Inc.

REFERENCES TO ADDITIONAL INFORMATION

This joint proxy statement/prospectus incorporates important business and financial information about First Financial Bancorp. (which we refer to as “First Financial”) and MainSource Financial Group, Inc. (which we refer to as “MainSource”) from documents filed with the Securities and Exchange Commission (which we refer to as the “SEC”) that are not included in or delivered with this joint proxy statement/prospectus. You can obtain any of the documents filed with or furnished to the SEC by First Financial and/or MainSource at no cost from the SEC’s website at http://www.sec.gov. You may also request copies of these documents, including documents incorporated by reference in this joint proxy statement/prospectus at no cost by contacting the appropriate company at the following address and phone number:

You will not be charged for any of these documents that you request. To obtain timely delivery of these documents, you must request them no later than five business days before the date of your meeting. This means that First Financial shareholders requesting documents must do so by [       ], in order to receive them before the First Financial special meeting, and MainSource shareholders requesting documents must do so by [       ], in order to receive them before the MainSource special meeting.

You should rely only on the information contained in, or incorporated by reference into, this document. No one has been authorized to provide you with information that is different from that contained in, or incorporated by reference into, this document. This document is dated [       ], and you should assume that the information in this document is accurate only as of such date. You should assume that the information incorporated by reference into this document is accurate as of the date of such document. Neither the mailing of this document to MainSource shareholders or First Financial shareholders, nor the issuance by First Financial of shares of common stock in connection with the merger, will create any implication to the contrary.

This document does not constitute an offer to sell, or a solicitation of an offer to buy, any securities, or the solicitation of a proxy, in any jurisdiction to or from any person to whom it is unlawful to make any such offer or solicitation in such jurisdiction. Except where the context otherwise indicates, information contained in this document regarding MainSource has been provided by MainSource and information contained in this document regarding First Financial has been provided by First Financial.

Please see “Where You Can Find More Information” for more details.

TABLE OF CONTENTS

QUESTIONS AND ANSWERS

The following are some questions that you may have about the merger and the First Financial special meeting or the MainSource special meeting, and brief answers to those questions. We urge you to read carefully the remainder of this joint proxy statement/prospectus because the information in this section does not provide all of the information that might be important to you with respect to the merger, the First Financial special meeting or the MainSource special meeting. Additional important information is also contained in the documents incorporated by reference into this joint proxy statement/prospectus. Please see “Where You Can Find More Information.”

you must obtain a legal proxy, executed in your favor, from the record holder of your shares, such as a broker, bank, or other nominee, to be able to vote in person at the meetings. If you only hold common stock in your capacity as a participant in the First Financial Savings Plan or the MainSource Plan, you may not instruct the respective trustee in person.

you will receive one or more separate proxy cards or voting instruction cards for each company. Please complete, sign, date, and return each proxy card and voting instruction card that you receive or otherwise follow the voting instructions set forth in this joint proxy statement/prospectus to ensure that you vote every share of First Financial common stock and/or MainSource common stock that you own.

SUMMARY

This summary highlights selected information from this joint proxy statement/prospectus. It may not contain all of the information that is important to you. We urge you to read carefully the entire joint proxy statement/prospectus, including the annexes, and the other documents to which we refer in order to fully understand the merger. Please see “Where You Can Find More Information.” Each item in this summary refers to the page of this joint proxy statement/prospectus on which that subject is discussed in more detail.

In the Merger, MainSource Shareholders Will Receive First Financial Common Stock (page 88)

First Financial and MainSource are proposing a strategic merger. If the merger is completed, MainSource shareholders will receive 1.3875 shares of First Financial common stock for each share of MainSource common stock they hold immediately prior to the merger. First Financial will not issue any fractional shares of First Financial common stock in the merger. MainSource shareholders who would otherwise be entitled to a fraction of a share of First Financial common stock upon the completion of the merger will instead receive, for the fraction of a share, an amount in cash (rounded to the nearest cent) based on the First Financial per share closing price.

As a result of the foregoing, based on the number of shares of First Financial and MainSource common stock outstanding as of July 25, 2017, on a fully diluted basis, approximately 63.4% and 36.6% of First Financial common stock will be held by First Financial shareholders and former MainSource shareholders, respectively, immediately following the effectiveness of the merger.

First Financial common stock is listed on NASDAQ under the symbol “FFBC,” and MainSource common stock is listed on NASDAQ under the symbol “MSFG.” The following table shows the closing sale prices of First Financial common stock and MainSource common stock as reported on NASDAQ on July 25, 2017, the last full trading day before the public announcement of the merger agreement, and on [         ], 2017, the last practicable trading day before the date of this joint proxy statement/prospectus. This table also shows the implied value of the merger consideration payable for each share of MainSource common stock, which was calculated by multiplying the closing price of First Financial common stock on those dates by the exchange ratio of 1.3875.

The merger agreement governs the merger. The merger agreement is included in this joint proxy statement/prospectus as Annex A. All descriptions in this summary and elsewhere in this joint proxy statement/prospectus of the terms and conditions of the merger are qualified by reference to the merger agreement. Please read the merger agreement carefully for a more complete understanding of the merger.

First Financial’s Board of Directors Unanimously Recommends that First Financial Shareholders Vote “FOR” the First Financial Merger Proposal and the Other Proposals Presented at the First Financial Special Meeting (page 48)

First Financial’s board of directors has determined that the merger, the merger agreement, and the transactions contemplated by the merger agreement are advisable and in the best interests of First Financial and its shareholders and has unanimously approved the merger agreement. First Financial’s board of directors unanimously recommends that First Financial shareholders vote “FOR” the adoption of the merger agreement and “FOR” the other proposals presented at the First Financial special meeting. For the factors considered by First Financial’s board of directors in reaching its decision to approve the merger agreement, see “The Merger— First Financial’s Reasons for the Merger; Recommendation of First Financial’s Board of Directors” beginning on page 48.

Each of First Financial’s directors, solely in his or her capacity as a First Financial shareholder, has entered into a voting agreement with MainSource, pursuant to which each such director has agreed to vote in favor of the First Financial merger proposal.

MainSource’s Board of Directors Unanimously Recommends that MainSource Shareholders Vote “FOR” the Adoption of the Merger Agreement and the Other Proposals Presented at the MainSource Special Meeting (page 63)

MainSource’s board of directors has determined that the merger, the merger agreement, and the transactions contemplated by the merger agreement are advisable and in the best interests of MainSource and its shareholders, and has unanimously approved the merger agreement. MainSource’s board of directors unanimously recommends that MainSource shareholders vote “FOR” the adoption of the merger agreement and “FOR” the other proposals presented at the MainSource special meeting. For the factors considered by MainSource’s board of directors in reaching its decision to approve the merger agreement, see “The Merger—MainSource’s Reasons for the Merger; Recommendation of MainSource’s Board of Directors” beginning on page 63.

Each of MainSource’s directors, solely in his or her capacity as a MainSource shareholder, has entered into a voting agreement with First Financial, pursuant to which each such director has agreed to vote in favor of the MainSource merger proposal.

Opinion of Sandler O’Neill & Partners, L.P. (page 50 and Annex B)

At the July 25, 2017 meeting at which the First Financial board of directors considered and approved the merger agreement, Sandler O’Neill & Partners, L.P. (which we refer to as “Sandler O’Neill”) delivered to the board of directors its oral opinion, which was subsequently confirmed in writing, to the effect that, as of such date, and subject to the procedures followed, assumptions made, matters considered, and qualifications and limitations described in Sandler O’Neill’s opinion, the exchange ratio provided for in the merger was fair to First Financial from a financial point of view.

The full text of Sandler O’Neill’s opinion is attached as Annex B to this joint proxy statement/prospectus. The opinion outlines the procedures followed, assumptions made, matters considered, and qualifications and limitations on the review undertaken by Sandler O’Neill in rendering its opinion.

First Financial shareholders are urged to read the entire opinion carefully in connection with their consideration of the proposed merger.

Sandler O’Neill’s opinion speaks only as of the date of the opinion. The opinion was directed to First Financial’s board of directors in connection with its consideration of the merger and is directed only to the fairness, from a financial point of view, of the exchange ratio to First Financial. Sandler O’Neill’s opinion does not constitute a recommendation to any First Financial shareholder as to how such First Financial shareholder should vote at any meeting of shareholders called to consider and vote upon the First Financial merger proposal. It does not address the underlying business decision of First Financial to engage in the merger, the form or structure of the merger, the relative merits of the merger as compared to any other alternative business strategies that might exist for First Financial, or the effect of any other transaction in which First Financial might engage.

For further information, see “The Merger—Opinion of Sandler O’Neill & Partners L.P.” beginning on page 50.

Opinion of Keefe, Bruyette & Woods, Inc. (page 66 and Annex C)

In connection with the merger, MainSource’s financial advisor, Keefe, Bruyette & Woods, Inc. (“KBW”), delivered a written opinion, dated July 25, 2017, to the MainSource board of directors as to the fairness, from a financial point of view and as of the date of the opinion, to the holders of MainSource common stock of the exchange ratio in the proposed merger. The full text of the opinion, which describes the procedures followed, assumptions made, matters considered, and qualifications and limitations on the review undertaken by KBW in preparing the opinion, is attached as Annex C to this joint proxy statement/prospectus. The opinion was for the information of, and was directed to, the MainSource board of directors (in its capacity as such) in connection with its consideration of the financial terms of the merger. The opinion did not address the underlying business decision of MainSource to engage in the merger or enter into the merger agreement or constitute a recommendation to the MainSource board of directors in connection with the merger, and it does not constitute a recommendation to any holder of MainSource common stock or any shareholder of any other entity as to how to vote in connection with the merger or any other matter.

For further information, see “The Merger—Opinion of Keefe, Bruyette & Woods, Inc.” beginning on page 66.

Treatment of MainSource Equity-Based Awards (page 89)

Options. At the effective time of the merger, each option granted by MainSource to purchase shares of MainSource common stock that is outstanding and unexercised immediately prior to the effective time, whether vested or unvested, shall without any further action on the part of the holder thereof, be assumed by First Financial and converted into an option to purchase from First Financial, on the same terms and conditions as were applicable under such MainSource options, a number of shares of First Financial common stock (rounded down to the nearest whole share) determined by multiplying (x) the number of shares of MainSource common stock subject to such MainSource stock option immediately prior to the effective time by (y) the exchange ratio, at a per share exercise price (rounded up to the nearest whole cent) equal to the quotient obtained by dividing (i) the per share exercise price for each share of MainSource common stock subject to such MainSource stock option by (ii) the exchange ratio.

Restricted Stock. Immediately prior to the effective time of the merger, each outstanding restricted share of MainSource common stock will fully vest (with any performance-based vesting condition deemed satisfied to the extent provided in the applicable award agreement) and be cancelled and converted automatically into the right to receive the merger consideration, less applicable tax withholdings.

Performance Share Units. Immediately prior to the effective time of the merger, each outstanding performance share unit award in respect of MainSource common stock will fully vest (with any performance-based vesting condition applicable to such performance share unit deemed satisfied at target) and shall be cancelled and converted automatically into the right to receive the merger consideration in respect of each share of MainSource common stock underlying such performance share unit, less applicable tax withholdings.

For further information, see “The Merger Agreement—Treatment of MainSource Equity-Based Awards” beginning on page 89.

First Financial Will Hold its Special Meeting on [          ] (page 31)

The First Financial special meeting will be held on [          ], at [          ] local time. Shareholders of First Financial will be able to attend the First Financial special meeting in person, at the Taft Centre at Fountain Square, 425 Walnut Street, Second Floor, Cincinnati, Ohio 45202, and will be able to view a webcast of the special meeting via the Internet at www.virtualshareholdermeeting.com/ffbc17SM. At the First Financial special meeting, First Financial shareholders will be asked to:

Only holders of record of First Financial common stock at the close of business on [          ] will be entitled to vote at the First Financial special meeting (which we refer to as the “First Financial record date”). Each share of First Financial common stock is entitled to one vote on each proposal to be considered at the First Financial special meeting. As of the First Financial record date, there were [          ] shares of First Financial common stock entitled to vote at the special meeting. The directors and executive officers of First Financial and their affiliates beneficially owned, and were entitled to vote, approximately [    ] shares of First Financial common stock, representing approximately [    ]% of the shares of First Financial common stock outstanding on the First Financial record date.

For further information, see “The First Financial Special Meeting—Date, Time, and Place of Meeting” beginning on page 31.

MainSource Will Hold its Special Meeting on [          ] (page 36)

The MainSource special meeting will be held on [          ], at [          ] local time, at 2105 North State Road 3 Bypass, Greensburg, Indiana 47240. At the MainSource special meeting, MainSource shareholders will be asked to:

Only holders of record of MainSource common stock at the close of business on [          ] will be entitled to vote at the MainSource special meeting (which we refer to as the “MainSource record date”). Each share of MainSource common stock is entitled to one vote on each proposal to be considered at the MainSource special meeting. As of the MainSource record date, there were [          ] shares of MainSource common stock entitled to vote at the special meeting. The directors and executive officers of MainSource and their affiliates beneficially owned, and were entitled to vote, approximately [          ] shares of MainSource common stock, representing approximately [    ]% of the shares of MainSource common stock outstanding on the MainSource record date.

For further information, see “The MainSource Special Meeting—Date, Time, and Place of Meeting” beginning on page 36.

First Financial Special Meeting Proposals: Required Vote; Treatment of Abstentions and Failure to Vote (page 31)

First Financial merger proposal:

Standard: Approval of the First Financial merger proposal requires the affirmative vote of the holders of at least two-thirds of the outstanding shares of First Financial common stock entitled to vote on the proposal.

Effect of abstentions and broker non-votes: If you fail to vote, mark “ABSTAIN” on your proxy card, or fail to instruct your bank or broker with respect to the First Financial merger proposal, it will have the same effect as a vote “AGAINST” the proposal.

First Financial adjournment proposal:

Standard: Approval of the First Financial adjournment proposal requires the affirmative vote of the holders of at least a majority of the votes present and entitled to vote at the First Financial special meeting.

Effect of abstentions and broker non-votes: If you mark “ABSTAIN” on your proxy card, it will have the same effect as a vote “AGAINST” the First Financial adjournment proposal. However, if you fail to submit a proxy card or vote in person at the First Financial special meeting, or fail to instruct your bank or broker how to vote with respect to the First Financial adjournment proposal, it will have no effect on the proposal.

For further information, see “The First Financial Special Meeting — Vote Required; Treatment of Abstentions and Failure to Vote” beginning on page 31.

MainSource Special Meeting Proposals: Required Vote; Treatment of Abstentions and Failure to Vote (page 36)

MainSource merger proposal:

Standard: Approval of the MainSource merger proposal requires the affirmative vote of the holders of at least a majority of the outstanding shares of MainSource common stock entitled to vote on the proposal.

Effect of abstentions and broker non-votes: If you fail to vote, mark “ABSTAIN” on your proxy, or fail to instruct your bank or broker with respect to the MainSource merger proposal, it will have the same effect as a vote “AGAINST” the proposal.

MainSource compensation proposal:

Standard: Approval of the MainSource compensation proposal requires the affirmative vote of the holders of at least a majority of the votes present and entitled to vote at the MainSource special meeting.

Effect of abstentions and broker non-votes: If you mark “ABSTAIN” on your proxy card, it will have the same effect as a vote “AGAINST” the MainSource compensation proposal. However, if you fail to submit a proxy card or vote in person at the MainSource special meeting, or fail to instruct your bank or broker how to vote with respect to the MainSource compensation proposal, it will have no effect on the proposal.

MainSource adjournment proposal:

Standard: Approval of the MainSource adjournment proposal requires the affirmative vote of the holders of at least a majority of the votes present and entitled to vote at the MainSource special meeting.

Effect of abstentions and broker non-votes: If you mark “ABSTAIN” on your proxy card, it will have the same effect as a vote “AGAINST” the MainSource adjournment proposal. However, if you fail to submit a proxy card

or vote in person at the MainSource special meeting, or fail to instruct your bank or broker how to vote with respect to the MainSource adjournment proposal, you will not be deemed to have cast a vote with respect to the proposal and it will have no effect on the proposal.

For further information, see “The MainSource Special Meeting—Vote Required; Treatment of Abstentions and Failure to Vote” beginning on page 36.

Material U.S. Federal Income Tax Consequences of the Merger (page 101)

It is a condition to the completion of the merger that First Financial and MainSource receive written opinions from their respective counsel to the effect that the merger will be treated as a “reorganization” within the meaning of Section 368(a) of the Code. Subject to the limitations and qualifications described in the section entitled “Material U.S. Federal Income Tax Consequences of the Merger,” if you are a U.S. holder of MainSource common stock, you will not recognize gain or loss pursuant to the merger, except in the case of cash received instead of fractional shares of First Financial common stock.

For a definition of “U.S. holder” and a more detailed discussion of the material United States federal income tax consequences of the merger, see the section entitled “Material U.S. Federal Income Tax Consequences of the Merger” beginning on page 100 of this joint proxy statement/prospectus.

The U.S. federal income tax consequences described above may not apply to all holders of MainSource common stock. We strongly urge you to consult your independent tax advisor for a full understanding of the application of U.S. federal income tax laws to your particular situation as well as any tax consequences arising under the U.S. federal estate or gift tax rules, or under the laws of any state, local, foreign, or other taxing jurisdiction or under any applicable treaty.

First Financial’s Officers and Directors Have Financial Interests in the Merger that Differ from Your Interests (page 77)

First Financial shareholders should be aware that First Financial’s directors and executive officers have interests in the merger that are different from, or in addition to, interests of First Financial shareholders generally. These interests include, among others, continued service as a director for certain First Financial directors, continued employment for certain executive officers of First Financial, and rights to ongoing indemnification and insurance coverage by the surviving corporation for acts or omissions occurring prior to the merger.

The First Financial board of directors was aware of and considered those interests, among other matters, in reaching its decisions to approve the merger agreement and the transactions contemplated thereby and to recommend the adoption of the merger agreement to First Financial shareholders. See the section entitled “The Merger—Interests of First Financial’s Directors and Executive Officers in the Merger” beginning on page 77 of this joint proxy statement/prospectus for a more detailed description of these interests.

MainSource’s Officers and Directors Have Financial Interests in the Merger that Differ from Your Interests (page 79)

MainSource’s shareholders should be aware that MainSource’s directors and executive officers have interests in the merger that are different from, or in addition to, interests of MainSource shareholders generally. These interests include, among others, the treatment of outstanding MainSource equity awards pursuant to the merger agreement, certain payments and benefits payable under change in control agreements entered into with certain officers, continued employment for certain executive officers of MainSource, and rights to ongoing indemnification and insurance coverage by the surviving corporation for acts or omissions occurring prior to the merger.

Pursuant to the merger agreement, First Financial has agreed to cause the number of directors that will comprise the board of directors of First Financial effective as of the effective time of the merger to be 15. Effective as of the effective time of the merger, the board of directors of First Financial shall consist of nine current members of the First Financial board of directors (the “First Financial Continuing Directors”) and six current members of the MainSource board of directors (the “MainSource Continuing Directors”). The First Financial Continuing Directors and the MainSource Continuing Directors shall be mutually agreed upon by the chairperson of the board of directors and Chief Executive Officer of First Financial and the lead director and Chief Executive Officer of MainSource.

Pursuant to the terms of the merger agreement, as of the date of the merger agreement, Claude E. Davis, Chief Executive Officer of First Financial, and Archie M. Brown, Jr., President and Chief Executive Officer of MainSource, entered into employment agreements which shall become effective at the effective time of the merger. Upon completion of the merger, Mr. Davis will become Executive Chairman and Mr. Brown will become President and Chief Executive Officer of the combined company. In addition, Chris M. Harrison, James M. Anderson, and Karen B. Woods, current executive officers of MainSource, will continue as officers of First Financial following the closing of the merger.

The MainSource board of directors was aware of and considered those interests, among other matters, in reaching its decisions to approve the merger agreement and the transactions contemplated thereby and to recommend the adoption of the merger agreement to MainSource shareholders. See the section entitled “The Merger—Interests of MainSource’s Directors and Executive Officers in the Merger” beginning on page 79 of this joint proxy statement/prospectus for a more detailed description of these interests.

MainSource Shareholders WILL NOT Be Entitled To Assert Dissenters’ Rights (page 85)

Under the Indiana Business Corporation Law (which we refer to as the “IBCL”), which is the law under which MainSource is incorporated, the holders of MainSource common stock will not be entitled to any dissenters’ rights in connection with the merger.

For more information, see “The Merger—Dissenters’ Rights in the Merger” beginning on page 85.

Regulatory Approvals Required for the Merger (page 85)

Subject to the terms of the merger agreement, both MainSource and First Financial have agreed to use their reasonable best efforts to obtain as promptly as practicable all regulatory approvals necessary or advisable to complete the transactions contemplated by the merger agreement and to comply with the terms and conditions of all such approvals. These approvals include approvals from, among others, the Board of Governors of the Federal Reserve System, which we refer to as the “Federal Reserve Board,” and the Ohio Division of Financial Institutions, which we refer to as the “ODFI.” First Financial and MainSource plan to file applications and notifications to obtain the required regulatory approvals.

Although neither MainSource nor First Financial knows of any reason why it cannot obtain these regulatory approvals in a timely manner, MainSource and First Financial cannot be certain when or if they will be obtained. For more information, see “The Merger—Regulatory Approvals Required for the Merger” beginning on page 85.

Conditions that Must Be Satisfied or Waived for the Merger to Occur (page 98)

Each party’s obligation to complete the merger is subject to the satisfaction or waiver (to the extent permitted under applicable law) of certain conditions, including:

Neither MainSource nor First Financial can be certain when, or if, the conditions to the merger will be satisfied or waived, or that the merger will be completed. For more information, see “The Merger Agreement—Conditions to Complete the Merger” beginning on page 98.

Termination of the Merger Agreement (page 98)

The merger agreement may be terminated at any time by First Financial or MainSource prior to the effective time of the merger under the following circumstances:

In addition, the merger agreement may be terminated:

For more information, see “The Merger Agreement—Termination of the Merger Agreement” beginning on page 98.

Termination Fee (page 100)

If the merger agreement is terminated under certain circumstances, including circumstances involving alternative acquisition proposals and changes in the recommendation of MainSource’s board of directors, MainSource may be required to pay to First Financial a termination fee equal to $40 million. This termination fee could discourage other companies from seeking to acquire or merge with MainSource. For more information, see “The Merger Agreement—Termination Fee” beginning on page 100.

Information About the Companies (page 41)

First Financial Bancorp.

First Financial is an Ohio corporation, organized in 1982, that owns all of the outstanding shares of common stock of First Financial Bank. At June 30, 2017, First Financial had, on a consolidated basis, $8.7 billion in

assets, $5.9 billion in loans, $6.5 billion in deposits, and shareholders’ equity of $0.9 billion. First Financial Bank, a growing full-service bank founded in 1863, is headquartered in Cincinnati, Ohio and is the principal bank subsidiary of First Financial. With $8.7 billion in assets and 1,429 full-time equivalent employees as of June 30, 2017, First Financial Bank accounts for substantially all of First Financial’s consolidated assets and results of operation. First Financial provides banking and financial services products through its four lines of business: Commercial and Private Banking, Retail Banking, Investment Commercial Real Estate, and Commercial Finance. These business units provide traditional banking services to business and retail clients including time and transaction deposit accounts, commercial loans, real estate loans, and consumer loans. Commercial and Private Banking includes First Financial Wealth Management, which provides wealth planning, portfolio management, trust and estate, brokerage, and retirement plan services.

First Financial’s principal office is located at 255 East Fifth Street, Suite 700, Cincinnati, Ohio 45202, and its telephone number at that location is (877) 322-9530. First Financial’s stock is traded on NASDAQ under the symbol “FFBC.” Additional information about First Financial and its subsidiaries is included in documents incorporated by reference in this joint proxy statement/prospectus. For more information, see “Information About First Financial” beginning on page 41 and “Where You Can Find More Information” beginning on page 124.

MainSource Financial Group, Inc.

MainSource is an Indiana corporation, organized in 1993, that owns all of the outstanding shares of common stock of MainSource Bank, which was established on January 1, 1904. At June 30, 2017, MainSource had, on a consolidated basis, $4.6 billion in assets, $3.0 billion in loans, $3.5 billion in deposits, and shareholders’ equity of $0.5 billion. MainSource Bank is headquartered in Greensburg, Indiana and is the principal bank subsidiary of MainSource. With $4.6 billion in assets and 979 full-time equivalent employees as of June 30, 2017, MainSource Bank accounts for substantially all of MainSource’s consolidated assets and results of operation. Within its geographic area, MainSource provides traditional banking products and services including time and transaction deposit accounts; consumer, commercial, agribusiness and real estate loans; corporate trust services; and other sophisticated financial products and services to business and retail clients.

MainSource’s principal office is located at 2105 North State Road 3 Bypass, Greensburg, Indiana 47240, and its telephone number at that location is (812) 663-6734. MainSource’s stock is traded on NASDAQ under the symbol “MSFG.” Additional information about MainSource and its subsidiaries is included in documents incorporated by reference in this joint proxy statement/prospectus. For more information, see “Information About MainSource” beginning on page 41 and “Where You Can Find More Information” beginning on page 124.

Risk Factors (page 25)

You should consider all the information contained in or incorporated by reference into this joint proxy statement/prospectus in deciding how to vote for the proposals presented in the joint proxy statement/prospectus. In particular, you should consider the factors described under “Risk Factors” beginning on page 25.

SELECTED CONSOLIDATED HISTORICAL FINANCIAL DATA OF FIRST FINANCIAL

The following table summarizes selected historical consolidated financial data of First Financial for the periods and as of the dates indicated. This information has been derived from First Financial’s consolidated financial statements filed with the SEC. Historical financial data as of and for the six months ended June 30, 2017 and June 30, 2016 are unaudited and include, in management’s opinion, all normal recurring adjustments considered necessary to present fairly the results of operations and financial condition of First Financial. You should not assume the results of operations for past periods and for the six months ended June 30, 2017 and 2016 indicate results for any future period.

You should read this information in conjunction with First Financial’s consolidated financial statements and related notes thereto included in First Financial’s Annual Report on Form 10-K for the year ended December 31, 2016, and in First Financial’s Quarterly Report on Form 10-Q for the six months ended June 30, 2017, which are incorporated by reference into this joint proxy statement/prospectus. See “Where You Can Find More Information” beginning on page 124 of this proxy statement/prospectus.

SELECTED CONSOLIDATED HISTORICAL FINANCIAL DATA OF MAINSOURCE

The following table summarizes selected historical consolidated financial data of MainSource for the periods and as of the dates indicated. This information has been derived from MainSource’s consolidated financial statements filed with the SEC. Historical financial data as of and for the six months ended June 30, 2017 and June 30, 2016 are unaudited and include, in management’s opinion, all normal recurring adjustments considered necessary to present fairly the results of operations and financial condition of MainSource. You should not assume the results of operations for past periods and for the six months ended June 30, 2017 and 2016 indicate results for any future period.

You should read this information in conjunction with MainSource’s consolidated financial statements and related notes thereto included in MainSource’s Annual Report on Form 10-K for the year ended December 31, 2016, and in MainSource’s Quarterly Report on Form 10-Q for the six months ended June 30, 2017, which are incorporated by reference into this joint proxy statement/prospectus. See “Where You Can Find More Information” beginning on page 124 of this proxy statement/prospectus.