A Maricopa County Superior Court spokesman says Washington entered the plea Monday.
Washington is charged with aggravated assault and criminal trespass. He has been free since he posted $2,400 bond after being arrested May 3.
Phoenix police have said Washington's ex-girlfriend claimed he grabbed her by the throat and shoved her to the ground following an argument at her apartment.
Washington led the Cardinals with 134 tackles and nine sacks last season. He signed a six-year contract extension in September.
He was suspended for the first four games of the 2013 season for violating the NFL's substance abuse policy.
Copyright 2013 by The Associated Press