Williams is the only sophomore among the semifinalists announced Tuesday.
Three finalists will be announced Nov. 25 and the winner will be announced Dec. 12.
Williams, a Freshman All-American last season, has a team-leading 54 tackles for the Trojans, who play at Oregon State on Friday night.
Williams has 11 tackles for losses, including five sacks.
Williams had eight tackles in USC's 19-3 victory over Utah last week.
UCLA linebacker Anthony Barr also is among the semifinalists.