Washington, 27, had a change of plea hearing in Maricopa County Superior Court and pleaded guilty to aggravated assault.
Sentencing is scheduled for April 23, but it wasn't immediately clear what he could be facing.
Washington has started 53 games over four seasons for Arizona. He had nine sacks last season and led the team in tackles in 2012 with 140.
A Cardinals spokesman said team officials would not be commenting on the Washington case because it is an ongoing legal matter.
Washington was suspended the first four games of last season for violating the NFL's substance abuse policy. He could be facing additional discipline for the assault under the league conduct policy.
Washington had been facing an April 30 trial in the case after originally pleading not guilty to aggravated assault and trespassing charges.
He was accused of grabbing his ex-girlfriend, the mother of the couple's daughter, by the throat and shoving her to the ground during an argument at her apartment last May.
His ex-girlfriend told Phoenix police that Washington pushed her with two hands, causing her to fall and break her right collarbone.