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 to fully understand the merger. 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.

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.

Information About the Companies (page 53)

Rockville Financial, Inc.

Rockville Financial, Inc. is a Connecticut corporation that owns all of the outstanding shares of common stock of Rockville Bank. At September 30, 2013, Rockville had, on a consolidated basis, assets of $2.22 billion, deposits of $1.69 billion and stockholders’ equity of $295.2 million. Rockville Bank, which is headquartered in Rockville, Connecticut, is a 22-branch community bank serving Tolland, Hartford and New London counties in Connecticut. Rockville Bank established a New Haven County Commercial Banking Office in Hamden, Connecticut, opened a full service Banking Center in West Hartford, Connecticut in January 2013 and opened a full service branch in Hamden, Connecticut in December 2013.

Rockville’s stock is traded on NASDAQ under the symbol “RCKB.”

Rockville’s principal office is located at 45 Glastonbury Boulevard, Suite 200, Glastonbury, Connecticut 06033, and its telephone number at that location is (860) 291-3600. Additional information about Rockville and its subsidiaries is included in documents incorporated by reference in this joint proxy statement/prospectus. See “Where You Can Find More Information,” beginning on page 142.

United Financial Bancorp, Inc.

United Financial Bancorp, Inc. is a Maryland corporation that owns all of the outstanding shares of common stock of United Bank. At September 30, 2013, United had, on a consolidated basis, assets of $2.49 billion, deposits of $1.95 billion and stockholders’ equity of $302.8 million. United Bank, a federal savings bank headquartered in West Springfield, Massachusetts, provides an array of financial products and services through its 16 branch offices and two express drive-up branches in the Springfield region of Western Massachusetts; seven branches in the Worcester region of Central Massachusetts; and 12 branches in Connecticut’s Hartford, Tolland and New Haven counties. United Bank also operates loan production offices located in Beverly, Massachusetts and Glastonbury, Connecticut. Through its Wealth Management Group, United Bank offers access to a wide range of investment and insurance products and services, as well as financial, estate and retirement strategies and products.

United’s stock is traded on NASDAQ under the symbol “UBNK.”

United’s principal office is located at 95 Elm Street, West Springfield, Massachusetts 01089, and its telephone number at that location is (413) 787-1700. Additional information about United and its subsidiaries is included in documents incorporated by reference in this joint proxy statement/prospectus. See “Where You Can Find More Information,” beginning on page 142.

In the Merger, United Common Stockholders Will Receive Shares of Rockville Common Stock (page 110)

Rockville and United are proposing a strategic merger. If the merger is completed, United common stockholders will receive 1.3472 shares of Rockville common stock for each share of United common stock they hold immediately prior to the merger. Rockville will not issue any fractional shares of Rockville common stock in the merger. United stockholders who would otherwise be entitled to a fraction of a share of Rockville common stock upon the completion of the merger will instead receive, for the fraction of a share, an amount in cash based on the Rockville closing share value. For example, if you hold 100 shares of United common stock, you will receive 134 shares of Rockville common stock and a cash payment instead of the 0.72 shares of Rockville common stock that you otherwise would have received (100 shares × 1.3472 = 134.72 shares).

Rockville common stock is listed on NASDAQ under the symbol “RCKB,” and United common stock is listed on NASDAQ under the symbol “UBNK.” The following table shows the closing sale prices of Rockville common stock and United common stock as reported on NASDAQ on November 14, 2013, the last trading day before the public announcement of the merger agreement, and on February 4, 2014, 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 United common stock, which we calculated by multiplying the closing price of Rockville common stock on those dates by the exchange ratio of 1.3472.

Rockville’s Board of Directors Unanimously Recommends that Rockville Stockholders Vote “FOR” the Approval of the Merger Agreement and the Other Proposals Presented at the Rockville Special Meeting (page 73)

Rockville’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 Rockville and its stockholders and has unanimously adopted the merger agreement. Rockville’s board of directors unanimously recommends that Rockville stockholders vote “FOR” the approval of the merger agreement and “FOR” the other proposals presented at the Rockville special meeting.

Rockville’s board of directors believes that United’s business and operations complement those of Rockville and that the merger would result in one of the premier community banks in New England, with approximately $4.8 billion in assets and over 50 banking locations from New Haven County, Connecticut to Springfield and Worcester, Massachusetts. The Rockville board considered that the combined company would have the scale and ability to deliver new and existing customers the best banking products and services, offer customers advanced banking technology solutions and provide them with more locations to do their banking. For additional factors considered by Rockville’s board of directors in reaching its decision to adopt the merger agreement, see “The Merger—Rockville’s Reasons for the Merger; Recommendation of Rockville’s Board of Directors.”

United’s Board of Directors Recommends that United Stockholders Vote “FOR” the Approval of the Merger Agreement and the Other Proposals Presented at the United Special meeting (page 59)

United’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 United and its stockholders and the directors in attendance at the November 14, 2013 board meeting voted unanimously to adopt the merger agreement. United’s board of directors recommends that United stockholders vote “FOR” the approval of the merger agreement and “FOR” the other proposals presented at the United special meeting.

United’s board of directors believes that the merger will provide value to United stockholders and create a larger and more diversified financial institution that is both better equipped to respond to economic and industry developments and better positioned to develop and build on its existing market position throughout Massachusetts and Connecticut. For additional factors considered by United’s board of directors in reaching its decision to adopt the merger agreement, see “The Merger—United’s Reasons for the Merger; Recommendation of United’s Board of Directors.”

Opinion of United’s Financial Advisor (page 62 and Annex B)

Opinion of Sterne, Agee & Leach, Inc.

In connection with its consideration of the merger, on November 14, 2013, the United board of directors received from Sterne, Agee & Leach, Inc., United’s financial advisor (which we refer to as “Sterne Agee”), its oral opinion, which opinion was confirmed by delivery of a written opinion, dated November 14, 2013, as to the fairness to United, from a financial point of view and as of the date of the opinion, of the exchange ratio provided for in the merger. The full text of Sterne Agee’s written opinion is attached as Annex B to this joint proxy statement/prospectus. You are urged to read the opinion in its entirety for a description of the procedures followed, assumptions made, matters considered, and qualifications and limitations on the review undertaken by Sterne Agee. The description of the opinion set forth herein is qualified in its entirety by reference to the full text of such opinion. Sterne Agee’s opinion speaks only as of the date of the opinion. The opinion is directed to the United board of directors and addresses only the fairness, from a financial point of view, of the exchange ratio offered to the United stockholders. It does not address the underlying business decision to proceed with the merger and does not constitute a recommendation to any United stockholder as to how the stockholder should vote at the United special meeting on the merger or any related matter.

For further information, see “The Merger—Opinion of Sterne Agee.”

Opinion of Rockville’s Financial Advisors (pages 74 and 80 and Annexes C and D)

Opinion of RBC Capital Markets, LLC

In connection with the merger, RBC Capital Markets, LLC (which we refer to as “RBCCM”), Rockville’s financial advisor, delivered to Rockville’s board of directors its oral opinion, which opinion was confirmed by delivery of a written opinion, dated November 14, 2013, as to the fairness to Rockville, from a financial point of view and as of the date of the opinion, of the exchange ratio provided for in the merger. The full text of RBCCM’s written opinion is attached as Annex C to this joint proxy statement/prospectus. You should read the entire opinion for a discussion of, among other things, the assumptions made, procedures followed, matters considered and limitations on the review undertaken by RBCCM in rendering its opinion. RBCCM’s opinion speaks only as of the date of the opinion. RBCCM delivered its opinion to Rockville’s board of directors for the benefit and use of Rockville’s board of directors (in its capacity as such) in connection with and for purposes of its evaluation of the merger. RBCCM’s opinion addressed only the exchange ratio from a financial point of view and did not address any other aspect of the merger and no opinion or view was expressed as to the relative merits of the merger in comparison to other strategies or transactions that might be available to Rockville or in which Rockville might engage or as to the underlying business decision of Rockville to proceed with or effect the merger. RBCCM’s opinion should not be construed as creating any fiduciary duty on the part of RBCCM to any party and does not address any other aspect of the merger and does not constitute a recommendation to any stockholder as to how to vote or act in connection with the proposed merger or any related matter.

Opinion of Sandler O’Neill + Partners, L.P.

In connection with the merger, Sandler O’Neill + Partners, L.P. (which we refer to as “Sandler O’Neill”) delivered a written opinion, dated November 14, 2013, to the Rockville board of directors as to the fairness, from a financial point of view and as of the date of such opinion, to Rockville of the merger consideration. The full text of Sandler O’Neill’s written opinion is attached as Annex D 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. The description of the opinion is qualified in its entirety by reference to the opinion. Rockville stockholders 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 the Rockville board of directors and is directed only to the fairness of the merger consideration to Rockville from a financial point of view. It does not address the underlying business decision of Rockville to engage in the merger or any other aspect of the merger and is not a recommendation to any Rockville stockholder as to how such stockholder should vote at the special meeting with respect to the merger or any other matter.

For further information, see “The Merger—Opinion of RBCCM” and “The Merger—Opinion of Sandler O’Neill.”

What Holders of United Stock Options and Other Equity-Based Awards Will Receive (page 111)

Restricted Stock. As of the effective time of the merger, the restrictions on each United restricted stock award will lapse, and the shares of United common stock underlying such awards will be treated as issued and outstanding shares of United common stock for all purposes under the merger agreement, including the payment of merger consideration.

Stock Options. At the effective time of the merger, each outstanding option to purchase shares of United common stock will become fully exercisable and be converted into an option to purchase Rockville common stock on the same terms and conditions as were applicable prior to the merger, except that (1) the number of shares of Rockville common stock subject to the new option will be equal to the product of the number of shares of United common stock subject to the existing option and the exchange ratio (rounding fractional shares down to the nearest whole share), and (2) the exercise price per share of Rockville common stock under the new option will be equal to the exercise price per share of United common stock of the existing option divided by the exchange ratio (rounded up to the nearest whole cent).

Rockville Will Hold its Special Meeting on April 8, 2014 (page 47)

The special meeting of Rockville stockholders will be held on Tuesday, April 8, 2014, at 10:00 a.m. local time, at Maneeley’s Banquet, Catering & Conference Center, 65 Rye Street, South Windsor, Connecticut 06074. At the special meeting, Rockville stockholders will be asked to approve:

Only holders of record at the close of business on January 31, 2014 will be entitled to vote at the special meeting. Each share of Rockville common stock is entitled to one vote on each proposal to be considered at the Rockville special meeting. As of the record date, there were 25,979,020 shares of Rockville common stock entitled to vote at the special meeting. As of the record date, the directors and executive officers of Rockville and their affiliates beneficially owned and were entitled to vote approximately 880,602 shares of Rockville common stock representing approximately 3.4% of the shares of Rockville common stock outstanding on that date.

United Will Hold its Special Meeting on April 8, 2014 (page 41)

The special meeting of United stockholders will be held on Tuesday, April 8, 2014, at 2:00 p.m. local time, at the Springfield Marriott, 6th Floor, 2 Boland Way, Springfield, Massachusetts 01115. At the special meeting, United stockholders will be asked to approve:

Only holders of record at the close of business on January 31, 2014 will be entitled to vote at the special meeting. Each share of United common stock is entitled to one vote on each proposal to be considered at the United special meeting. As of the record date, there were 19,784,428 shares of United common stock entitled to vote at the special meeting. As of the record date, the directors and executive officers of United and their affiliates beneficially owned and were entitled to vote approximately 698,169 shares of United common stock representing approximately 3.5% of the shares of United common stock outstanding on that date.

Required Vote; Treatment of Abstentions and Failure to Vote

Rockville merger proposal:

Rockville adjournment proposal and Rockville compensation proposal:

Rockville certificate amendment proposal:

United merger proposal:

United adjournment proposal and United compensation proposal:

The Merger Is Generally Tax-Free to Holders of United Common Stock as to the Shares of Rockville Common Stock They Receive (page 125)

The merger is intended to qualify as a “reorganization” within the meaning of Section 368(a) of the Internal Revenue Code of 1986, as amended (which we refer to as the “Code”), and tax counsel for each of Rockville and United has provided a legal opinion to that effect. Such opinions have been filed as exhibits to the registration statement of which this joint proxy statement/prospectus is a part. Accordingly, unless the holder is subject to special treatment under the tax laws, the merger will be tax-free to a holder of United common stock for U.S. federal income tax purposes as to the shares of Rockville 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 Rockville common stock that such holder of United common stock would otherwise be entitled to receive.

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

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

Rockville’s Officers and Directors Have Interests in the Merger that Differ from Your Interests (page 90)

Rockville stockholders should be aware that some of Rockville’s directors and executive officers have interests in the merger and have arrangements that are different from, or in addition to, those of Rockville stockholders generally. Rockville’s board of directors was aware of these interests and considered these interests, among other matters, when making its decision to adopt the merger agreement, and in recommending that Rockville stockholders vote in favor of approving the merger agreement.

These interests include the following:

The following table sets forth the aggregate consideration payable to each of Rockville’s directors and executive officers in connection with the merger, assuming: (1) the effective time of the merger occurred on December 31, 2013, (2) a per share price of Rockville’s common stock of $14.72, and (3) in the case of executive officers, their employment is terminated without cause on December 31, 2013. However, Rockville anticipates that the executive officer payments will not be triggered in all cases except for Messrs. Koniecki, Lund and Trachimowicz. The amounts shown are based on multiple assumptions that may or may not occur, including assumptions described in this joint proxy statement/prospectus, and do not reflect certain compensation actions that may occur before the completion of the merger.

The amounts shown for non-employee directors above reflect the value of acceleration of the vesting of restricted stock and stock options to each director. For a more complete description of these interests, see “The Merger—Interests of Rockville’s Directors and Executive Officers in the Merger.”

United’s Officers and Directors Have Interests in the Merger that Differ from Your Interests (page 99)

United stockholders should be aware that some of United’s directors and executive officers have interests in the merger and have arrangements that are different from, or in addition to, those of United stockholders generally. United’s board of directors was aware of these interests and considered these interests, among other matters, when making its decision to adopt the merger agreement, and in recommending that United stockholders vote in favor of approving the merger agreement.

These interests include the following:

The following table sets forth the amount of payments and benefits that may be paid or become payable to each of the named executive officers and directors of United in connection with the merger, assuming: (1) the effective time of the merger occurred on December 31, 2013 and (2) a per share price of United common stock of $19.38, the average closing price per share over the first five business days following the announcement of the merger agreement. The amounts shown below are estimates based on multiple assumptions that may or may not actually occur, including assumptions described in this joint proxy statement/prospectus, and do not reflect certain compensation actions that may occur before the completion of the merger. As a result, the actual amounts to be received by a named executive officer or director may differ materially from the amounts set forth below.

The amounts set forth below assume that employment of each of Messrs. Collins and Roberts is terminated without cause following the closing of the merger. However, United anticipates that the change in control benefits, other than the acceleration of “single trigger” restricted stock awards and options and SERP benefits, for Messrs. Sullivan and Valade will not be triggered as each will continue to be employed with the resulting company following the merger.

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

United Stockholders Are NOT Entitled To Assert Dissenters’ Rights (page 107)

Under the Maryland General Corporation Law (which we refer to as the “MGCL”), which is the law under which United is incorporated, the holders of United common stock will not be entitled to any appraisal rights or dissenters’ rights in connection with the merger. For more information, see “The Merger—Dissenters’ Rights in the Merger.”

Conditions that Must Be Satisfied or Waived for the Merger To Occur (page 120)

Currently, United and Rockville expect to complete the merger in the first half of 2014. As more fully described in this joint proxy statement/prospectus and in the merger agreement, the completion of the merger depends on a number of conditions being satisfied or, where legally permissible, waived. These conditions include:

Neither United nor Rockville can 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 121)

The merger agreement can be terminated at any time prior to completion of the merger in the following circumstances:

Termination Fee (page 122)

If the merger agreement is terminated under certain circumstances involving a competing offer, United or Rockville may be required to pay to the other party a termination fee equal to $15,000,000.

Amendments to Rockville’s Certificate of Incorporation (page 105 and Annex E)

In connection with the merger, Rockville’s certificate of incorporation will be amended at the effective time of the merger to change the name of the surviving corporation to “United Financial Bancorp, Inc.” Approval of the merger agreement by Rockville stockholders also constitutes approval of the foregoing amendment to Rockville’s certificate of incorporation, which we refer to as the “name change amendment.”

In addition, the merger agreement requires Rockville to submit to its stockholders the Rockville certificate amendment proposal, which would set the number of directors of the surviving corporation in accordance with the surviving corporation’s bylaws (effectively raising the number from 16 to 20) and reclassify the surviving corporation board of directors from a four-class board with staggered four-year terms to a three-class board with staggered three-year terms. Although Rockville agrees to use its reasonable best efforts to obtain stockholder approval of the Rockville certificate amendment proposal (which requires the approval of not less than 80% of the outstanding shares of Rockville common stock), stockholder approval of the Rockville certificate amendment proposal is not a condition to the closing of the merger.

A copy of the proposed amendment of the certificate of incorporation is attached hereto as Annex E to this joint proxy statement/prospectus.

Corporate Governance Agreements and Amendment to Rockville’s Bylaws (page 105 and Annex F)

On November 14, 2013, the board of directors of Rockville adopted a resolution amending Rockville’s bylaws, effective as of the effective time of the merger. We refer to this amendment as the “bylaw amendment,” a copy of which is attached as Annex F to this joint proxy statement/prospectus. The bylaw amendment provides:

Under the merger agreement, the board of directors of the surviving bank at the effective time will be constituted in the same manner and with the same individuals as the board of directors of the surviving corporation and subject to the same requirements set forth in the bylaw amendment. However, the board of directors of the surviving bank will consist of 20 members equally divided between the parties whereas the surviving corporation will have 20 members only if the Rockville certificate amendment proposal passes. If the Rockville certificate amendment proposal does not pass, the board of directors of the surviving corporation will consist of 16 members equally divided between the parties.

Employment and Advisory Agreements (pages 92, 101 and 102)

Simultaneous with the execution of the merger agreement, Rockville entered into employment agreements with each of Messrs. Crawford and Sullivan, to be effective as of and subject to the completion of the merger. These agreements set forth the terms and conditions of each such individual’s employment relationship with

Rockville following the effective time of the merger and, when effective, supersede and replace any prior employment, retention, change of control or other similar agreement with such individual. Also, simultaneous with the execution of the merger agreement, Rockville entered into an advisory agreement with Mr. Collins, which advisory agreement is effective as of and subject to the completion of the merger.

For more detail on the terms of these employment and advisory agreements, see “The Merger—Interests of Rockville’s Directors and Executive Officers in the Merger” and “The Merger—Interests of United’s Directors and Executive Officers in the Merger.”

Regulatory Approvals Required for the Merger (page 107)

To complete the transactions contemplated by the merger agreement, Rockville and United must receive approvals from, among others, the Federal Reserve Board, the FDIC, the Connecticut Banking Commissioner and the BBI. Notice to the OCC of the merger of United Bank with and into Rockville Bank is also required, although no approval is required in connection with such notice. Rockville and United have filed applications and notifications to obtain the required regulatory approvals.

Although neither Rockville nor United knows of any reason why it cannot obtain these regulatory approvals in a timely manner, Rockville and United cannot be certain when or if they will be obtained.

The Rights of United Stockholders Will Change as a Result of the Merger (page 130)

The rights of United stockholders will change as a result of the merger due to differences in Rockville’s and United’s governing documents and states of incorporation. The rights of United stockholders are governed by Maryland law and by United’s articles of incorporation and bylaws, each as amended to date. Upon the completion of the merger, United stockholders will become stockholders of Rockville, as the continuing legal entity in the merger, and the rights of United stockholders will therefore be governed by Connecticut law and Rockville’s certificate of incorporation and bylaws.

See “Comparison of Stockholders’ Rights” for a description of the material differences in stockholders’ rights under each of the Rockville and United governing documents.

Risk Factors (page 36)

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.”

RECENT DEVELOPMENTS

Rockville

The following presents a summary of Rockville’s financial results for the three months and years ended December 31, 2013 and 2012.

Rockville Financial, Inc.

Condensed Consolidated Statements of Net Income

(Unaudited)

The highlights as of and for the three months and year ended December 31, 2013 included the following:

United

The following presents a summary of United’s financial results for the three months and years ended December 31, 2013 and 2012.

United Financial Bancorp, Inc.

Condensed Consolidated Statements of Net Income

(Unaudited)

The highlights as of and for the three months and year ended December 31, 2013 included the following:

SELECTED CONSOLIDATED HISTORICAL FINANCIAL DATA OF ROCKVILLE

The following selected consolidated financial information for the years ended December 31, 2008 through December 31, 2012 is derived from audited financial statements of Rockville. The financial information as of and for the nine months ended September 30, 2013 and 2012 is derived from unaudited financial statements and, in the opinion of Rockville’s management, reflects all adjustments, consisting only of normal recurring adjustments, necessary for a fair presentation of this data for those dates. The results of operations for the nine months ended September 30, 2013 are not necessarily indicative of the results that may be expected for the entire year ending December 31, 2013. You should not assume the results of operations for any past periods indicate results for any future period. You should read this information in conjunction with Rockville’s consolidated financial statements and related notes thereto included in Rockville’s Annual Report on Form 10-K for the year ended December 31, 2012, in Rockville’s Quarterly Report on Form 10-Q for the quarter ended September 30, 2013 and in other prior filings made with the SEC, which are incorporated by reference into this joint proxy statement/prospectus. See “Where You Can Find More Information.”

SELECTED CONSOLIDATED HISTORICAL FINANCIAL DATA OF UNITED

The following selected consolidated financial information for the years ended December 31, 2008 through December 31, 2012 is derived from audited financial statements of United. The financial information as of and for the nine months ended September 30, 2013 and 2012 is derived from unaudited financial statements and, in the opinion of United’s management, reflects all adjustments, consisting only of normal recurring adjustments necessary for a fair presentation of this data for those dates. The results of operations for the nine months ended September 30, 2013 are not necessarily indicative of the results that may be expected for the entire year ending December 31, 2013. You should not assume the results of operations for any past periods indicate results for any future period. You should read this information in conjunction with United’s consolidated financial statements and related notes thereto included in United’s Annual Report on Form 10-K for the year ended December 31, 2012, in United’s Quarterly Report on Form 10-Q for the quarter ended September 30, 2013 and in other prior filings made with the SEC, which are incorporated by reference into this joint proxy statement/prospectus. See “Where You Can Find More Information.”