Williams, a 6-foot-5, 300-pound junior, has 8 1/2 tackles for losses, including six sacks. He also has intercepted a pass and forced three fumbles.
Other finalists: Ohio State's Joey Basa, Clemson's Vic Beasley, Washington's Hau'oli Kikaha, Utah's Nate Orchard, Missouri's Shane Ray.
The winner will be announced Dec. 10.
Williams is regarded as a top pro prospect. He is expected to make himself available for the 2015
USC finished the regular season with an 8-4 record. The Trojans will learn their bowl destination on Sunday.