Jackson, who played at Gardena Serra High, started the last eight games for the Trojans. He made 42 tackles, including four for losses. He broke up nine passes and forced a fumble.
Jackson also played receiver, catching seven passes, two for touchdowns.
He averaged 27.7 yards per kickoff return, which ranks fourth nationally, and scored on a 100-yard return against Utah.
Jackson is the third USC player to be recognized as freshman defensive player of the year. Linebacker Dion Bailey won in 2011, defensive lineman Leonard Williams in 2012. Safety Taylor Mays was freshman of the year in 2006, before the conference recognized players separately for offense and defense.