SUMMARY

This summary highlights selected information from this proxy statement/prospectus. It may not contain all of the information that is important to you. We urge you to carefully read the entire proxy statement/prospectus, including the appendices, and the other documents to which we refer in order to fully understand the merger. See “Where You Can Find More Information” on page 159. Each item in this summary refers to the page of this proxy statement/prospectus on which that subject is discussed in more detail.

In the Merger, BancTrust Shareholders Will Have a Right to Receive 0.125 of a Share of Trustmark Common Stock Per Share of BancTrust Common Stock (page 73)

We are proposing the merger of BancTrust with and into Trustmark, with Trustmark continuing as the surviving corporation of the merger. If the merger is completed, you will have the right to receive 0.125 of a share of Trustmark common stock for each share of BancTrust common stock you hold immediately prior to the merger. Trustmark will not issue any fractional shares of Trustmark common stock in the merger. BancTrust shareholders who would otherwise be entitled to a fractional share of Trustmark common stock will instead receive an amount in cash based on the last reported sale price of Trustmark common stock on the trading day immediately prior to the date on which the merger is completed.

Example: If you hold 150 shares of BancTrust common stock, you will have a right to receive 18 shares of Trustmark common stock and a cash payment instead of the 0.75 shares of Trustmark common stock that you otherwise would have received (i.e., 150 shares x 0.125 = 18.75 shares).

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

BancTrust’s Board of Directors Unanimously Recommends that BancTrust Shareholders Vote “FOR” Approval of the Merger Agreement (page 54)

BancTrust’s board of directors has determined that the merger agreement and the transactions contemplated by the merger agreement are advisable and in the best interests of BancTrust and its shareholders and has unanimously approved and adopted the merger agreement and the transactions contemplated thereby (including the merger). BancTrust’s board of directors unanimously recommends that BancTrust shareholders vote “FOR” approval of the merger agreement. For the factors considered by BancTrust’s board of directors in reaching its decision to approve and adopt the merger agreement, see the section entitled “The Merger—BancTrust’s Reasons for the Merger; Recommendation of the BancTrust Board of Directors” beginning on page 54.

Keefe, Bruyette & Woods, Inc. Has Provided an Opinion to BancTrust’s Board of Directors Regarding the Merger Consideration (page 56 and Annex B)

On May 28, 2012, Keefe, Bruyette & Woods, Inc., which is referred to as KBW, BancTrust’s financial advisor in connection with the merger, rendered its oral opinion to BancTrust’s board of directors, and subsequently confirmed in writing, that as of such date and based upon and subject to the assumptions, procedures, considerations, qualifications and limitations set forth in the written opinion, the exchange ratio pursuant to the merger agreement was fair, from a financial point of view, to the holders of shares of BancTrust common stock.

The full text of KBW’s opinion, dated May 28, 2012, is attached as Annex B to this proxy statement/prospectus. You should read the opinion in its entirety for a discussion of the assumptions made, procedures followed, factors considered and limitations upon the review undertaken by KBW in rendering its opinion.

KBW’s opinion is directed to BancTrust’s board of directors, addresses only the fairness, from a financial point of view, of the exchange ratio pursuant to the merger agreement, and does not address any other aspect of the merger or constitute a recommendation as to how any shareholders of BancTrust should vote at any shareholder’s meeting held in connection with the merger.

For further information, please see the discussion under the caption “The Merger—Opinion of Keefe, Bruyette & Woods, Inc.,” beginning on page 56.

What Holders of BancTrust Stock Options and Other Equity-Based Awards Will Receive (page 73)

Each outstanding and unvested option to purchase shares of BancTrust common stock will vest in full and become exercisable 30 days prior to the effective time of the merger. Any BancTrust stock options that are not exercised on or before the effective time of the merger will terminate and be cancelled for no consideration.

Each outstanding share of BancTrust restricted stock will vest in full and become free of restrictions immediately prior to the effective time of the merger, and at the effective time of the merger will be converted into the right to receive 0.125 of a share of Trustmark common stock.

Additionally, the deferred stock rights in respect of BancTrust common stock under BancTrust’s amended and restated directors deferred compensation plan will be converted into the right to receive the number of shares of Trustmark common stock equal to the number of shares of BancTrust common stock underlying the deferred stock right multiplied by the exchange ratio, with distributions to be made in accordance with the terms of the plan.

BancTrust will Hold its Special Meeting on September 26, 2012 (page 44)

The BancTrust special meeting will be held on September 26, 2012, at 10:00 a.m., local time, at BancTrust’s corporate offices located at 107 Saint Francis Street, Mobile, Alabama 36602. At the BancTrust special meeting, BancTrust shareholders will be asked to:

Only holders of record at the close of business on August 10, 2012 will be entitled to vote at the BancTrust special meeting. Each share of BancTrust common stock is entitled to one vote on each proposal to be considered at the BancTrust special meeting. As of the record date, there were 17,967,388 shares of BancTrust common stock entitled to vote at the BancTrust special meeting. As of the record date, directors and executive officers of BancTrust and their affiliates owned and were entitled to vote 1,776,528 shares of BancTrust common stock, representing approximately 9.89 percent of the shares of BancTrust common stock outstanding on that date. BancTrust currently expects that its directors and executive officers will vote their shares in favor of the merger agreement proposal, the merger-related named executive officer compensation proposal and the adjournment proposal, although none of them has entered into any agreements obligating them to do so. As of the record date, Trustmark beneficially held no shares of BancTrust common stock, and Trustmark’s directors and executive officers and their affiliates held no shares of BancTrust common stock.

To approve the merger agreement, holders of at least two-thirds of the outstanding shares of BancTrust common stock entitled to vote at the BancTrust special meeting must vote in favor of approving the merger agreement. Because approval is based on the affirmative vote of at least two-thirds of the shares outstanding, your failure to vote, failure to instruct your bank or broker with respect to the proposal to approve the merger agreement, or abstention will have the same effect as a vote against approval of the merger agreement.

Approval of the merger-related named executive officer compensation proposal requires the affirmative vote of more shares in favor of the proposal than against the proposal. Because approval of the merger-related named executive officer compensation proposal is based on the affirmative vote of shares voting at the BancTrust special meeting, your abstention, failure to vote or failure to instruct your bank or broker with respect to the merger-related named executive officer compensation proposal will have no effect on this proposal.

Approval of the adjournment proposal requires the affirmative vote of the holders of a majority of the shares of BancTrust common stock present, in person or by proxy, at the BancTrust special meeting, even if less than a quorum. Because approval of the adjournment proposal is based on the affirmative vote of a majority of shares voting or expressly abstaining at the BancTrust special meeting, abstentions will have the same effect as a vote against the adjournment proposal. The failure to vote or failure to instruct your bank or broker with respect to the adjournment proposal, however, will have no effect on the adjournment proposal.

The Merger Is Intended to Be Tax-Free to Holders of BancTrust Common Stock as to the Shares of Trustmark Common Stock They Receive (page 89)

The merger is intended to qualify as a “reorganization” within the meaning of Section 368(a) of the Code, and, as a condition to the respective obligations of Trustmark and BancTrust to complete the merger, each of Trustmark and BancTrust shall receive a legal opinion to that effect. Accordingly, the merger generally will be tax-free to a holder of BancTrust common stock for U.S. federal income tax purposes as to the shares of Trustmark common stock he or she receives in the merger, except for any gain or loss that may result from the receipt of cash instead of fractional shares of Trustmark common stock that such holder of BancTrust common stock would otherwise be entitled to receive.

For further information, see “Material U.S. Federal Income Tax Consequences of the Merger” beginning on page 89.

The U.S. federal income tax consequences described above may not apply to all holders of BancTrust common stock. Your tax consequences will depend on your individual situation. Accordingly, we strongly urge you to consult your tax advisor for a full understanding of the particular tax consequences of the merger to you.

BancTrust’s Directors and Officers May Have Financial Interests in the Merger That Differ From Your Interests (page 64)

BancTrust shareholders should be aware that some of BancTrust’s directors and executive officers may have interests in the merger and have arrangements that may be different from, or in addition to, those of BancTrust shareholders generally. These interests and arrangements may create potential conflicts of interest. The BancTrust board of directors was aware of these interests and considered these interests, among other matters, when making its decision to approve and adopt the merger agreement and the merger, and in recommending that BancTrust’s shareholders vote in favor of approval of the merger agreement.

For a more complete description of these interests, see “The Merger—Interests of BancTrust’s Directors and Executive Officers in the Merger” beginning on page 64.

BancTrust’s Shareholders May Exercise Dissenters’ Rights (page 68)

Under the Alabama Business Corporation Law, which is referred to as the ABCL, holders of BancTrust common stock, if they follow the required procedures, have the right to dissent from the merger and receive the fair market value of their shares of BancTrust common stock as determined by a court. BancTrust shareholders exercising their dissenters’ rights must comply strictly with the procedures specified in Article 13 of the ABCL. See the section entitled “The Merger—Dissenters’ Appraisal Rights” beginning on page 68 and the text of Article 13 of the ABCL attached to this proxy statement/prospectus as Annex C.

Treatment of BancTrust TARP Preferred Stock and BancTrust TARP Warrant in the Merger (page 74)

Trustmark intends to purchase each issued and outstanding share of Fixed Rate Cumulative Perpetual Preferred Stock, Series A, no par value and liquidation preference of $1,000 per share, which is referred to as the BancTrust TARP preferred stock, from the United States Department of the Treasury, which is referred to as Treasury, prior to the closing of the merger. Trustmark also intends to purchase the warrant issued by BancTrust to Treasury on December 19, 2008 to purchase up to 731,000 shares of BancTrust’s common stock at an exercise price of $10.26 per share, which is referred to as the BancTrust TARP warrant, prior to the closing of the merger.

Regulatory Approvals Required for the Merger (page 70)

We have agreed to use our reasonable best efforts to obtain all regulatory approvals required to complete the transactions contemplated by the merger agreement; however, in no event will Trustmark be required to raise common equity capital at the holding company level in an amount which would reduce the economic benefits of the transactions contemplated by the merger agreement to Trustmark to such a degree that Trustmark would not have entered into the merger agreement had such condition to raise common equity capital been known to it at the date of the merger agreement, which is referred to as a burdensome condition. These approvals include approval from the Board of Governors of the Federal Reserve System, which is referred to as the Federal Reserve Board, and from the Office of the Comptroller of the Currency, which is referred to as the OCC, among others. Trustmark and BancTrust have filed, or are in the process of filing, applications and notifications to obtain the required regulatory approvals.

Although we do not know of any reason why we cannot obtain these regulatory approvals in a timely manner, we cannot be certain when or if we will obtain them.

Conditions That Must Be Satisfied or Waived for the Merger to Occur (page 86)

Currently, we expect to consummate the merger in the fourth quarter of 2012. As more fully described in this proxy statement/prospectus and in the merger agreement, consummation of the merger depends on a number of conditions being satisfied or, where legally permissible, waived. The conditions to each party’s obligation to complete the merger include, among others:

We cannot be certain when, or if, the conditions to the merger will be satisfied or waived, or that the merger will be completed.

Termination of the Merger Agreement (page 87)

We may mutually agree to terminate the merger agreement before completing the merger, even after receiving BancTrust shareholder approval.

In addition, either of us may decide to terminate the merger agreement if:

In addition, Trustmark may terminate the merger agreement if BancTrust’s board of directors:

Termination Fee (page 88)

If the merger agreement is terminated under certain circumstances, including circumstances involving a change in recommendation by BancTrust’s board of directors, BancTrust may be required to pay Trustmark a termination fee of $5 million. The termination fee could discourage other companies from seeking to acquire or merge with BancTrust.

Board of Directors and Executive Officers of Trustmark and Trustmark National Bank following the Effective Time of the Merger (page 64)

The directors and officers of Trustmark immediately prior to the effective time of the merger will continue as the directors and officers of the surviving corporation of the merger. As of the effective time of the merger, two current directors of BancTrust who are mutually selected by BancTrust and Trustmark will be appointed as directors of Trustmark National Bank.

The Rights of BancTrust Shareholders will Change as a Result of the Merger (page 101)

The rights of BancTrust shareholders will change as a result of the merger due to differences in Trustmark’s and BancTrust’s governing documents. The rights of BancTrust shareholders are governed by Alabama law and by BancTrust’s amended and restated articles of incorporation and amended and restated bylaws, each as amended to date (which are referred to as BancTrust’s articles of incorporation and bylaws, respectively). Upon the effective time of the merger, the rights of BancTrust shareholders will be governed by Mississippi law and Trustmark’s articles of incorporation and bylaws (which are referred to as Trustmark’s articles of incorporation and bylaws, respectively).

This proxy statement/prospectus contains descriptions of the material differences in shareholder rights under each of the BancTrust and Trustmark articles of incorporation and bylaws.

Comparative Market Prices of Securities (page 18)

Trustmark common stock and BancTrust common stock are listed on NASDAQ under the symbols “TRMK” and “BTFG,” respectively. The following table presents the closing prices of Trustmark common stock and BancTrust common stock on March 21, 2012, the last trading day before BancTrust’s announcement of its intention to pursue a strategic transaction, on May 25, 2012, the last trading day before public announcement of the merger, and on August 10, 2012, the last practicable trading day before the distribution of this proxy statement/prospectus. The table also presents the implied value of the merger consideration proposed for each share of BancTrust common stock on those dates, as determined by multiplying the closing price of Trustmark common stock on those dates by the exchange ratio of 0.125 provided for in the merger agreement, and assuming no adjustment.

Litigation Relating to the Merger (page 71)

BancTrust, BancTrust’s board of directors, and Trustmark were named as defendants in a lawsuit filed in connection with the merger. The lawsuit challenges the fairness of the merger and seeks various forms of relief, including an injunction barring the defendants from consummating the merger. See “The Merger—Litigation Relating to the Merger” beginning on page 71 for more information.

Trustmark Corporation

Trustmark, a Mississippi business corporation incorporated in 1968, is a bank holding company headquartered in Jackson, Mississippi. Trustmark’s principal subsidiary is Trustmark National Bank, initially chartered by the State of Mississippi in 1889. At March 31, 2012, Trustmark National Bank had total assets of $9.8 billion, which represents over 99 percent of the consolidated assets of Trustmark.

Through Trustmark National Bank and its other subsidiaries, Trustmark operates as a financial services organization providing banking and other financial solutions through approximately 170 offices and 2,611 full-time equivalent associates located in the states of Mississippi, Tennessee (in Memphis and the Northern Mississippi region), Florida (primarily in the northwest or “Panhandle” region of that state) and Texas (primarily in Houston). Trustmark National Bank provides investment and insurance products and services to its customers through its wholly owned subsidiaries Trustmark Investment Advisors, Inc. and Fisher Brown Bottrell Insurance, Inc.

As of March 31, 2012, on a consolidated basis, Trustmark had total assets of approximately $9.9 billion, total loans of approximately $6.2 billion, total deposits of approximately $8.1 billion and shareholders’ equity of approximately $1.2 billion.

Trustmark’s principal executive office is located at 248 East Capitol Street Jackson, Mississippi 39201, and its telephone number is (601) 208-5111.

Additional information about Trustmark and its subsidiaries is included in documents incorporated by reference in this proxy statement/prospectus. See “Where You Can Find More Information” on page 159.

BancTrust Financial Group, Inc.

BancTrust is an Alabama corporation headquartered in Mobile, Alabama and was incorporated in 1986. BancTrust is registered under the Bank Holding Company Act of 1956, as amended, and its principal subsidiary is BankTrust, an Alabama banking corporation.

At March 31, 2012, BancTrust had total assets of approximately $2.009 billion, total consolidated deposits of approximately $1.791 billion and total consolidated shareholders’ equity of approximately $112.6 million and ranked third in terms of total assets among Alabama-based bank holding companies.

BancTrust’s principal executive offices are located at 107 Saint Francis Street, Mobile, Alabama 36602, and its telephone number is (251) 431-7800.

Additional information about BancTrust and its subsidiaries is included under the section “Information About BancTrust” on page 27.