MAI MANAGED VOLATILITY FUND (the “Fund”)

Supplement dated June 30, 2026 to the Prospectus dated December 30, 2025

In April 2026, MAI Capital Management, LLC (“Adviser”), the investment adviser to the Fund, communicated to the Board of Trustees (“Board”) of Forum Funds (“Trust”) that an affiliate of The Carlyle Group Inc. (“Carlyle”) had entered into a definitive agreement to acquire MAI Capital Management Holdings LLC, a Delaware limited liability company and the parent entity of the Adviser, via merger (the “Transaction”). The Transaction would cause the Adviser to experience a change of control, resulting in the automatic termination of the original investment advisory agreement between the Trust and the Adviser (the “Original Agreement”).

In anticipation of the change in control of the Adviser, and to provide for continuity of management, at a meeting held on May 13, 2026, the Board appointed the Adviser pursuant to an interim advisory agreement with the Trust, on behalf of the Fund (the “Interim Agreement”), effective upon the close of the Transaction. The Interim Agreement allows the Adviser to continue serve as the investment adviser to the Fund under terms identical to those provided under the Original Agreement, including the compensation to be paid to the Adviser, except that the Interim Agreement will remain in effect until the earlier of (i) 150 days from the close of the Transaction on June 4, 2026, and (ii) the date that the Fund’s shareholders approve a new investment advisory agreement for the Fund.

On June 26, 2026, the Board also approved a new investment advisory agreement between the Trust, on behalf of the Fund, and the Adviser (the “New Agreement”) and determined to submit the New Agreement to the Fund’s shareholders for their approval. The New Agreement will replace the Interim Agreement if the shareholders approve the New Agreement at the special meeting of shareholders scheduled to take place on or about September 14, 2026.

Additional information regarding the New Agreement is expected to be provided in connection with the solicitation of Fund shareholder approval of the New Agreement, including a proxy statement to be sent to shareholders of the Fund, as of a specified record date.

The Transaction is not expected to result in any change to the Fund’s portfolio management team, investment objective, principal investment strategy or investment policies. Under the Interim Agreement and the New Agreement, the Adviser will receive the same compensation that it would have received from the Fund under the Original Agreement. The Board also approved the renewal of the expense cap/reimbursement agreement on behalf of the Fund, which will keep the Fund’s expenses at the same levels as provided under the previous expense cap/reimbursement agreement with the Adviser.

As a result of the changes described above, the Prospectus is supplemented as follows:

The section entitled “Investment Adviser” on page 14 of the Prospectus is hereby deleted in its entirety and replaced with the following:

Investment Adviser

Effective June 4, 2026, MAI Capital Management, LLC (“Adviser”) was appointed to serve as the interim investment adviser to the Fund pursuant to an Interim Investment Advisory Agreement (“Interim Agreement”) entered into between the Trust, on behalf of the Fund, and the Adviser. The Interim Agreement will remain in effect for no more than 150 days from June 4, 2026 or until Fund shareholders approve a new investment advisory agreement, whichever is earlier.

The Adviser is located at 6050 Oak Tree Blvd, Suite 500, Cleveland, OH 44131. The Adviser is a registered investment adviser under the Investment Advisers Act of 1940 and has provided investment advisory and management services to clients since 1973.

Subject to the general oversight of the Board, the Adviser makes investment decisions for the Fund. The Adviser receives an annual advisory fee from the Fund at an annual rate equal to 0.75% of the Fund’s average annual daily net assets under the terms of the Interim Agreement and would be paid at an annual rate equal to 0.75% of the Fund’s average annual daily net assets under the New Agreement, if approved by the shareholders. The Adviser has contractually agreed to waive its fee and/or reimburse Fund expenses to limit the Fund’s Total Annual Fund Operating Expenses (excluding all taxes, interest, dividends on short sales, acquired fund fees and expenses, brokerage costs, and extraordinary expenses) of Investor Class to 1.24%, and Institutional Class to 0.99% through at least September 30, 2028 (“Expense Cap”).

The Expense Cap may only be raised or eliminated with the consent of the Board. The Adviser may be reimbursed by the Fund for fees waived and expenses reimbursed by the Adviser pursuant to the Expense Cap if such payment is made within three years of the fee waiver or expense reimbursement and does not cause the Total Annual Fund Operating Expenses After Fee Waiver and/or Expense Reimbursement of the Fund to exceed the lesser of (i) the then-current expense cap and (ii) the expense cap in place at the time the fees/expenses were waived/reimbursed. Total Annual Fund Operating Expenses will increase if exclusions from the Expense Cap apply.

A discussion summarizing the basis on which the Board approved the New Agreement will be included in the Fund’s reports on Form N-CSR.

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For more information, please contact a Fund customer service representative at (877) 414-7884 (toll free).

PLEASE RETAIN FOR FUTURE REFERENCE.

MAI MANAGED VOLATILITY FUND (the “Fund”)

Supplement dated June 30, 2026 to the Statement of Additional Information (“SAI”)

dated December 30, 2025

In anticipation of the change in control of the Adviser, and to provide for continuity of management, at a meeting held on May 13, 2026, the Board appointed the Adviser pursuant to an interim advisory agreement with the Trust, on behalf of the Fund (the “Interim Agreement”), effective upon the close of the Transaction. The Interim Agreement allows the Adviser to continue serve as the investment adviser to the Fund under terms identical to those provided under the Original Agreement, except that the Interim Agreement will remain in effect until the earlier of (i) 150 days from the close of the Transaction on June 4, 2026, and (ii) the date that the Fund’s shareholders approve a new investment advisory agreement for the Fund.

As a result of the changes described above, the SAI is supplemented as follows:

In the “Board of Trustees, Management and Service Providers” section of the SAI, in the sub-section entitled “F. Investment Adviser,” the paragraphs entitled “Services of Adviser,” “Ownership of Adviser” and “Advisory Agreement” beginning on page 26 are hereby deleted in their entirety and replaced with the following:

Services of Adviser. Effective June 4, 2026, MAI Capital Management, LLC (“Adviser”) was appointed to serve as the interim investment adviser to the Fund pursuant to an Interim Investment Advisory Agreement (“Interim Agreement”) entered into between the Trust, on behalf of the Fund, and the Adviser. The Interim Agreement will remain in effect for no more than 150 days from June 4, 2026 or until Fund shareholders approve a new investment advisory agreement, whichever is earlier.

Under the Interim Agreement, the Adviser furnishes, at its own expense, all services, facilities and personnel necessary in connection with managing the Fund’s investments and effecting portfolio transactions for the Fund. The Adviser may compensate brokers or other service providers (“Financial Intermediaries”) out of its own assets, and not as additional charges to the Fund, in connection with the sale and distribution of shares of the Fund and/or servicing of these shares.

Ownership of Adviser. The Adviser is owned by MAI Capital Management Holdings LLC, an affiliate of The Carlyle Group Inc. Richard J. Buoncore is the Executive Chairman of the Adviser and responsible for its day-to-day operations.

Advisory Agreement. Effective June 4, 2026, the Board appointed the Adviser to serve as the interim investment adviser to the Fund and commence management of the Fund pursuant to the Interim Agreement. The Interim Agreement will remain in effect for no more than 150 days from June 4, 2026 or until Fund shareholders approve a new investment advisory agreement, whichever is earlier.

The Interim Agreement is terminable, without penalty, on 10 days’ written notice by the Board or by a vote of a majority of the voting securities of the Fund, or by the Adviser on 60 days’ written notice to the Trust. The Interim Agreement terminates immediately in the event of its assignment. Under the Interim Agreement, the Adviser is not liable for any mistake of judgment or mistake of law, or any event whatsoever except for lack of good faith or by reason of bad faith, willful misfeasance, or gross negligence in the performance of its duties or by reason of reckless disregard of its obligations and duties under the Interim Agreement.

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For more information, please contact a Fund customer service representative at (877) 414-7884 (toll free).

PLEASE RETAIN FOR FUTURE REFERENCE.