An inmate at the Roxbury Correctional Institution was assaulted Thursday afternoon during an outdoor recreation period at the prison south of Hagerstown.
"The staff responded immediately and summoned medical help for the inmate," said Division of Correction spokeswoman Maxine Eldridge.
The inmate was taken to Washington County Hospital where he was under observation, Eldridge said.
All inmates who were in the prison yard for recreation were returned to their cells, she said.
An investigation was launched but no information was available Thursday. Eldridge said no one was in custody for the attack.
Eldridge wouldn't release the inmate's name Thursday because his family hadn't been informed of the incident.