Williams, a two-time All-America selection, had 80 total tackles this season, the second-most on the team. He also tied for the team lead with seven sacks and had an interception.
Williams had nine tackles and a sack in his final college game, a victory over Nebraska in Saturday's Holiday Bowl.
The 6-foot-5, 300-pound Williams could play as either a defensive end or defensive tackle as a pro. Prior to the start of this season, NFL.com ranked Williams as the No. 3 draft prospect.
"This was a very hard decision for me," Williams said in a statement. "I love USC, Coach Sark and the coaching staff, my teammates and the fans, and I love the game of college football. It's hard to leave all of that. I know life is never the same once you leave college. But I feel like it's the right time for me to go. I feel like I'm ready. I'm looking forward to the new challenge that the NFL presents."
- Dan Gartland