Matthew Joseph Medlin, 30, was described as 5 feet, 8 inches tall, 185 pounds with a bald head and several facial tattoos.
At the time of his escape from Columbia River Correctional Institute, around 12:37 p.m. Wednesday, he was wearing blue jeans and a blue T-shirt with the words "Department of Corrections" stenciled on it.
Medlin was due to be released from prison in one week after being sentenced to jail for burglary, sex abuse and assault.
Portland police spokesman Sgt. Pete Simpson said he didn't know why Medlin would have chosen to escape days before his legal release.
Police said Medlin may have cut himself after climbing over a fence and may need medical attention.