UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM

CURRENT REPORT

PURSUANT TO SECTION 13 OR 15(d)

OF THE SECURITIES EXCHANGE ACT OF 1934

Date of Report (Date of Earliest Event Reported): July 16, 2026

(Exact Name of Registrant as Specified in its Charter)

19-01

Route 208 North, Fair Lawn, New Jersey 07410

(Address of principal executive offices)

(800)

(Registrant’s telephone number, including area code)

Check the appropriate box below if the Form filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Securities registered pursuant to Section 12(b) of the Act:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule of the Securities Exchange Act of 1934 of this chapter).

Emerging growth company ☐

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 13(a) of the Exchange Act. ☐

EXPLANATORY NOTE

On July 20, 2026, Columbia Bank MHC (the “MHC”) completed its conversion from a mutual holding company to a stock holding company (the “Conversion”) and Columbia Financial, Inc., a Maryland corporation (the “Company”), completed its stock offering conducted in connection with the Conversion, all pursuant to a Plan of Conversion and Reorganization dated as of January 31, 2026 (the “Plan”).

Upon completion of the Conversion, the Company became the holding company for Columbia Bank (the “Bank”) and acquired ownership of all the issued and outstanding capital stock of the Bank. In connection with the Conversion, 167,236,353 shares of common stock, par value $0.01 per share, of the Company (the “Common Stock”) were sold in a subscription offering and firm commitment underwritten offering (together, the “Offering”) at a price of $10.00 per share, for gross proceeds of $1.7 billion. In addition and in accordance with the Plan, approximately 61,686,775 additional shares of Common Stock were issued to the public shareholders ( stockholders other than the MHC) of Columbia Financial, Inc., a Delaware corporation and the former holding company for the Bank (the Holding Company”), as of the closing date of the Conversion in exchange for their outstanding shares of common stock of the Holding Company (the “Exchange”). Each such share of common stock of the Holding Company was converted into the right to receive 2.2000 shares of Common Stock.

Immediately following the completion of the Conversion, the Company also completed its previously announced acquisition of Northfield Bancorp, Inc. (“Northfield Bancorp”), the parent company of Northfield Bank (the “Merger”), pursuant to the Agreement and Plan of Merger, dated as of January 31, 2026, by and among the Company, the Holding Company, the MHC and Northfield Bancorp (the “Merger Agreement”). At the effective time of the Merger, each share of Northfield Bancorp common stock was converted into the right to receive either $14.25 in cash or 1.425 shares of Common Stock, or a combination thereof, subject to the elections made by Northfield Bancorp stockholders and the proration procedures set forth in the Merger Agreement.

The Common Stock issued in the Offering was registered under the Securities Act of 1933, as amended (the “Securities Act”), pursuant to a Registration Statement on Form (File initially filed with the Securities and Exchange Commission (“SEC”) on March 6, 2026, as amended, and declared effective by the SEC on May 11, 2026 (the “Form The Common Stock issued in the Exchange and the Merger was registered under the Securities Act pursuant to a Registration Statement on Form (File initially filed with the SEC on March 6, 2026, as amended, and declared by the SEC on May 11, 2026 (the “Form

The Common Stock is deemed registered under Section 12(b) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), pursuant to subsection (a) of Rule promulgated under the Exchange Act as a result of the Company becoming the successor issuer to the Holding Company in connection with the Conversion. The description of the Common Stock set forth under the heading in the prospectus included in the Form is incorporated herein by reference.

On July 16, 2026, the Company, the Holding Company, the MHC and the Bank entered into an Underwriting Agreement, dated as of July 16, 2026, with Keefe, Bruyette & Woods, Inc., (“KBW”), as representative of the underwriters named in the Underwriting Agreement, in connection with the firm commitment underwritten offering undertaken by the Company as part of the Conversion. Subject to the terms and conditions of the Underwriting Agreement, the Underwriters agreed to purchase an aggregate of 52,291,781 shares of Common Stock at a price of $9.62 per share. Under the Underwriting Agreement, in the event of a firm commitment underwritten offering with transaction proceeds between $500 million and $700 million in connection with the Conversion, the underwriters are entitled to receive an underwriting discount not to exceed the greater of (x) 3.5% of the aggregate purchase price of the shares of Common Stock sold in the firm commitment underwritten offering or (y) a percentage discount resulting in an aggregate discount, expressed as a dollar amount, of $20,000,000.

The foregoing description of the terms of the Underwriting Agreement is qualified in its entirety by reference to the Underwriting Agreement, which is filed as Exhibit 1.1 hereto and incorporated by reference herein.

On July 16, 2026, the Company, the Holding Company, the MHC, the Bank and KBW entered into Amendment No. 1 (the “Agency Agreement Amendment”) to the Agency Agreement, dated as of May 11, 2026, by and between the Company, the Holding Company, the MHC, the Bank and KBW (the “Agency Agreement”). The Agency Agreement Amendment amended the Agency Agreement to provide that, in the event of a firm commitment underwritten offering with transaction proceeds between $500 million and $700 million in connection with the Conversion, the underwriters will be entitled to receive an underwriting discount not to exceed the greater of (x) 3.5% of the aggregate purchase price of the shares of Common Stock sold in the firm commitment underwritten offering or (y) a percentage discount resulting in an aggregate discount, expressed as a dollar amount, of $20,000,000.

The foregoing description of the terms of the Underwriting Agreement is qualified in its entirety by reference to the Agency Agreement Amendment, which is filed as Exhibit 1.2 hereto and incorporated by reference herein.

On July 20, 2026, immediately following the completion of the Conversion, the Company consummated the Merger in accordance with the terms of the Merger Agreement. At the effective time of the Merger, each share of Northfield Bancorp common stock was converted into the right to receive either $14.25 in cash or 1.425 shares of Common Stock, or a combination thereof, subject to the elections made by Northfield Bancorp stockholders and the proration procedures set forth in the Merger Agreement. Immediately following the completion of the Merger, Northfield Bank was merged with and into Columbia Bank, with Columbia Bank continuing as the surviving institution (the “Bank Merger”).

The foregoing description of the Merger, the Bank Merger and the Merger Agreement does not purport to be complete and is qualified in its entirety by reference to the full text of the Merger Agreement, a copy of which is filed hereto as Exhibit 2.1 and incorporated herein by reference.

In accordance with the Merger Agreement, at the effective time of the Merger, the Company assumed all of Northfield Bancorp’s obligations required by that certain Indenture (the “Indenture”), dated as of June 17, 2022, with respect to Northfield Bancorp’s $62,000,000 principal amount of 5.00% rate subordinated notes due 2032 (the “2022 Notes”).

The supplemental indentures pursuant to which the Company assumed the Indenture pursuant to which the 2022 Notes were issued has not been filed herewith pursuant to Item 601(b)(4)(v) of Regulation under the Securities Act. The Company agrees to furnish a copy of such supplemental indenture, and the Indenture, to the SEC upon request.

In accordance with the terms of the Merger Agreement, at the effective time of the Merger, the size of the Board of Directors of the Company and the Bank was increased by four members and four directors of Northfield Bancorp — Steven M. Klein, John P. Connors, Jr., Timothy C. Harrison and Paul V. Stahlin (collectively, the “Legacy Northfield Bancorp Directors”) — were appointed as directors of the Company and the Bank effective as of the effective time of the Merger. In addition, Steven M. Klein, the Chairman, President and Chief Executive Officer of Northfield Bancorp, was appointed as Senior Executive Vice President and Chief Operating Officer of the Company and the Bank effective as of the effective time of the Merger.

The information regarding the Legacy Northfield Bancorp Directors included under the heading in the prospectus included in the Form is incorporated herein by reference.

On January 20, 2026, the Company issued a press release announcing the completion of the Conversion and the Merger. A copy of the press release is filed as Exhibit 99.1 to this Current Report on Form and is incorporated herein by reference.

The financial statements of Northfield Bancorp included under the heading in the prospectus included in the Form are incorporated herein by reference.

The pro forma financial information included under the heading in the prospectus included in the Form is incorporated herein by reference.

SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, as amended, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunder duly authorized.