COLB · Columbia Banking System, Inc.

ESPP Purchase

The deferred compensation plan phantom stock was accrued under the Registrant's deferred compensation plan and will be settled in accordance with the terms of the deferred compensation plan following the reporting person's cessation of service as a director of the Registrant.

Beneficial ownership has been adjusted to correct a prior reporting error, as a disposition of 3,000 shares reported on August 29, 2024 was not reflected in subsequent ownership totals.

The deferred compensation plan phantom stock was accrued under the Registrant's deferred compensation plan and will be paid in accordance with the terms of the deferred compensation plan following the end of the reporting person's employment with the Registrant.

Restricted Stock Units convert into common stock on a one-for-one basis. On February 25, 2025, the reporting person was granted 8,458 Restricted Stock Units, which vest in three annual installments beginning on March 13, 2026.

Restricted Stock Units convert into common stock on a one-for-one basis. On February 25, 2025, the reporting person was granted 15,037 Restricted Stock Units, which vest in three annual installments beginning on March 13, 2026.

Restricted Stock Units convert into common stock on a one-for-one basis. On February 25, 2025, the reporting person was granted 54,511 Restricted Stock Units, which vest in three annual installments beginning on March 13, 2026.

Restricted Stock Units convert into common stock on a one-for-one basis. On February 25, 2025, the reporting person was granted 4,699 Restricted Stock Units, which vest in three annual installments beginning on March 13, 2026.

Restricted Stock Units convert into common stock on a one-for-one basis. On February 25, 2025, the reporting person was granted 2,819 Restricted Stock Units, which vest in three annual installments beginning on March 13, 2026.

Restricted Stock Units convert into common stock on a one-for-one basis. On February 25, 2025, the reporting person was granted 7,142 Restricted Stock Units, which vest in three annual installments beginning on March 13, 2026.

This Form 4/A amends the Form 4 filed on March 17, 2026 solely to correct the transaction code for the reported disposition from S to F. The transaction represents shares withheld by the issuer to satisfy tax obligations in connection with equity award vesting.

This Form 4/A amends the Form 4 filed on March 17, 2026 solely to correct the transaction code for the reported disposition from S to F. The transaction represents shares withheld by the issuer to satisfy tax obligations in connection with equity award vesting.

Restricted Stock Units convert into common stock on a one-for-one basis. On February 25, 2025, the reporting person was granted 7,518 Restricted Stock Units, which vest in three annual installments beginning on March 13, 2026.

Restricted Stock Units convert into common stock on a one-for-one basis. On February 25, 2025, the reporting person was granted 15,037 Restricted Stock Units, which vest in three annual installments beginning on March 13, 2026.

Open market sale pursuant to a 10B5-1 Plan which was established by Reporting Person on November 25, 2025, in accordance with Rule 10B5-1 of the Securities Exchange Act of 1934, as amended.

Open market sale pursuant to a 10B5-1 Plan which was established by Reporting Person on November 25, 2025, in accordance with Rule 10B5-1 of the Securities Exchange Act of 1934, as amended.