Over 13 games this season, Allen rushed for a team-high 1,489 yards and 11 TDs while adding 458 receiving yards on 41 catches.
The 6-foot-1, 220-pound redshirt junior led the Pac-12 with 149.8 all-purpose yards per game and ranked third with 114.5 yards rushing per game.
He earned first-team all-conference honors.
"This was a tough decision for me," Allen said in the statement. "I went home to Florida and talked it over with my family. I am proud to have been the first one in my family to have gone to college, and I've been blessed to say I played football in college. Now I'm ready to take it to the next level. Every kid dreams of this moment.
"This has been a long journey. There are so many I want to thank, starting with God, who made this all possible. I want to thank President Nikias for the opportunity I had at this great university and all of those who impacted my life at USC, including all our fans. I want to thank Pat Haden and J.K. McKay for accepting me and always being there for me. I want to thank Coach O for trusting in me when he took over last year and Coach Tommie Robinson for putting his job on the line last year by giving me the chance to play. And now Coach Sark and his staff for molding me as a man and as a complete person on and off the field. I had a great year with Coach Sark and his staff. I also appreciate all the help I got from our equipment and training staffs, who met all my needs. And especially everyone in our Academic Services department, who did such a great job supporting me and making my experience at USC better every day. Fight On!"