Officials at the Jackie Brannon Correctional Center in McAlester said inmate Glen C. Washington, 47, was reported missing.
The correctional center officers conducted two counts and confirmed at 3:20 p.m. that Washington was missing.
Corrections Emergency Response Teams and K-9 units were initiated, authorities said.
Washington is serving a 20-year sentence for possession of a controlled dangerous substance out of Atoka County.
According to the Oklahoma Department of Corrections, Washington is a black male, 6'1 and weighs 180 pounds. Washington has black hair and brown eyes.