NOTE 1 - DESCRIPTION OF PLAN

The following description of the Prosperity Bancshares, Inc. 401(k) Profit Sharing Plan (the “Plan”) provides only general information. Participants should refer to the Plan’s document for a more complete description of the Plan’s provisions.

A. General

The Plan is a defined contribution plan covering all full-time and part-time employees of Prosperity Bank (the “Bank”), a wholly owned subsidiary of Prosperity Bancshares, Inc., who have completed at least three (3) months of service and are twenty-one (21) years of age or older. An employee’s entry date is the first day of the month coinciding with or next following the date they satisfy the eligibility requirements. The Plan is subject to the provisions of the Employee Retirement Income Security Act of 1974 (“ERISA”).

B. Contributions

Each year a participant may contribute up to 75% of eligible compensation subject to the maximum amount allowable under the Internal Revenue Code of 1986, as amended (the “Code”). Participants also have the option of making Roth salary deferral contributions. If a participant is age fifty (50) or older, he or she may elect to defer additional amounts as catch-up contributions. Participants may change their contribution percentage on the beginning of each payroll period or stop contributing at any time. Participants are also permitted to deposit into the Plan, distributions from other plans and certain Individual Retirement Accounts as rollover contributions.

The Bank, at its discretion, may contribute to the Plan a matching contribution which is determined annually. In 2025, the Bank matched fifty percent (50%) of the participants’ contributions subject to certain limitations, excluding catch-up contributions, up to fifteen percent (15%) of their eligible compensation, calculated annually. The Bank may also make a discretionary profit sharing contribution. No profit sharing contributions were made during 2025.

C. Participant Accounts

Each participant’s account is credited with the participant’s contributions and allocations of (a) employer matching contributions and profit sharing contributions, and (b) Plan earnings (losses), and is charged with an allocation of administrative expenses. Allocations are based on participant earnings or account balances, as defined. The benefit to which a participant is entitled is the benefit that can be provided from the participant’s vested account. At December 31, 2025 and 2024, all investments of the Plan were participant-directed.

D. Vesting

Participants are immediately vested in their contributions plus actual earnings (losses) thereon. Vesting in the employer matching contribution and profit sharing contribution of participant accounts plus actual earnings (losses) thereon is based on years of continuous service. To qualify for a year of service for vesting purposes, the participant must complete one thousand (1,000) hours of service in that calendar year. Participants vest twenty percent (20%) per year after two (2) years of service and are one hundred percent (100%) vested after six (6) years of service. A participant becomes 100% vested when terminated from employment due to death or disability, or upon reaching the normal retirement age of 62.

E. Notes Receivable from Participants

Participants may borrow from their fund accounts a minimum of $1,000 up to a maximum equal to the lesser of $50,000 or fifty percent (50%) of their vested account balance. Loan terms generally range from 1 - 5 years, but can be longer if the loan is used to purchase a principal residence. The loans are secured by the balance in the participant’s account and bear interest at a rate commensurate with the local prevailing rates. Principal and interest are paid ratably through scheduled payroll deductions. A participant may have up to two outstanding loans at a time.

NOTE 2 - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

NOTE 3 - FAIR VALUE MEASUREMENTS

The Plan utilizes the provisions of Accounting Standards Codification (“ASC”) 820, Fair Value Measurement, with respect to its investments. ASC 820 defines fair value as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants at the measurement date. ASC 820 establishes a three-level fair value hierarchy that prioritizes the inputs used to measure fair value. This hierarchy requires entities to maximize the use of observable inputs and minimize the use of unobservable inputs. The three levels of inputs used to measure fair value are as follows:

NOTE 4 – RISKS AND UNCERTAINTIES

The Plan provides for various investment options of Prosperity Bancshares, Inc. common stock, mutual funds, fixed income securities, collective investment trusts, and other investment securities. Investment securities, in general, are exposed to various risks, such as interest rate, credit, and overall market volatility risk. Due to the level of risk associated with certain investment securities, it is reasonably possible that changes in the values of investment securities will occur in the near term and that such changes could materially affect the amounts reported in the statements of net assets available for benefits and the amounts reported in participant accounts.

The Plan's investment in shares of Prosperity Bancshares, Inc. common stock represents 10% and 11% of total investments at fair value as of December 31, 2025 and 2024, respectively. As a result of this concentration, any significant fluctuation in the market value of the common stock could affect individual participant accounts and the net assets available for benefits of the Plan.

NOTE 5 - TAX STATUS

The Plan adopted a prototype non-standardized pre-approved defined contribution plan document, which obtained its latest opinion letter dated June 30, 2020 in which the IRS stated that the prototype document satisfies the applicable provisions of the Code. The Bank believes that the Plan is designed and is currently being operated in compliance with the applicable requirements of the Code. Therefore, contributions to the Plan and earnings from its assets are not currently taxable to the Plan or its participants.

GAAP requires plan management to evaluate uncertain tax positions taken by the Plan. The financial statement effects of a tax position are recognized when the position is more likely than not, based on the technical merits, not to be sustained upon examination by the IRS. The plan administrator has analyzed the tax positions taken by the Plan, and has concluded that as of December 31, 2025 and 2024, there are no uncertain positions taken or expected to be taken. The Plan has recognized no interest or penalties related to uncertain tax positions. The Plan is subject to routine audits by taxing jurisdictions. There are currently no audits in progress.

NOTE 6 - PARTY-IN-INTEREST TRANSACTIONS

The Plan allows transactions with certain parties who may perform services or have fiduciary responsibilities to the Plan, including the Bank and Fidelity Management Trust Company ("Fidelity"), the trustee of the Plan. The Plan invests in common stock of Prosperity Bancshares, Inc. and a money market account at the Bank, and certain investments in mutual funds and collective investment trusts managed by entities affiliated with Fidelity. These transactions, as well as notes receivable from participants qualify as party-in-interest transactions. These transactions are covered by an exemption from the prohibited transaction provisions.

NOTE 7 - SUBSEQUENT EVENTS

Effective January 1, 2026, Prosperity Bancshares, Inc. (the “Company”) completed the merger of American Bank Holding Corporation (“American”) with and into the Company and the subsequent merger of American’s wholly owned subsidiary American Bank, N.A., into the Bank. In conjunction with the transaction, accounts of the acquired employees participating in their previous employer's qualified plan were transferred into the Plan on April 1, 2026. On April 1, 2026, $53,678,675 was transferred into the Plan.

Effective February 1, 2026, the Company completed the merger of Southwest Bancshares, Inc. (“Southwest”) into the Company and the subsequent merger of Southwest’s wholly owned subsidiary, Texas Partners Bank, into the Bank. In conjunction with the transaction, the Texas Partners Bank 401(k) Plan was terminated. Associates have the option to join the Plan.

On January 28, 2026, the Company and Stellar Bancorp, Inc. (“Stellar”) jointly announced the signing of a definitive merger agreement whereby Stellar, the parent company of Stellar Bank, will merge with and into the Company and Stellar Bank will merge with and into the Bank. In conjunction with the pending transaction, accounts of the acquired employees participating in their previous employer's qualified plan are expected to be transferred into the Plan during the third quarter 2026, but the amount of plan assets to be transferred is currently being valued.

Management has evaluated subsequent events from December 31, 2025 through June 22, 2026, the date the financial statements were available to be issued, and found no subsequent events, other than those noted above.

* A party-in-interest as defined by ERISA.

** Cost information is omitted, as these accounts are participant directed.