A court clerk confirmed Friday that Ellison entered the plea Wednesday in Los Angeles County Superior Court in Torrance. He remains free on $10,000 bail.
The 23-year-old former All-Pac-10 safety and USC defensive captain was released by the Chargers after he was pulled over for speeding by police in Redondo Beach May 24. He was arrested after officers allegedly found he had about 100 Vicodin pills but no prescription for the painkiller.
The Seahawks and former USC coach Pete Carroll then claimed Ellison off waivers.
The arrest has prompted a Drug Enforcement Administration investigation of Dr. David Chao on the Chargers' medical staff.