The Guardians said the policy is in accordance with the federal vaccination mandate, which the Biden administration announced in September.
The rule states places with 100 or more employees mandate the vaccine or start weekly testing. The deadline is Jan. 4. Those who do not comply may face termination.
The Guardians, which officially made the switch from Cleveland Indians last week, did not release any additional information.
The Houston Astros and the Washington Nationals were the first MLB teams to require the COVID vaccine for all non-playing, full-time employees in August.