Prosecutors said Mark E. Thom, 31, pleaded guilty Friday. Plea documents said Thom beat and used a stun gun on a handcuffed man in December of 2011.
U.S. Attorney David J. Hickton said Thom admitted using excessive force against a non-resisting victim.
Springdale Borough officials said Thom had been on leave and has now resigned.
Thom was also sued last year by an Allegheny County police officer who contends Thom drew a gun on him and, along with other Springdale officers, falsely accused the county officer of crimes during a February 2011 traffic dispute.
Sentencing is scheduled for July 12.