Farmer had 25 catches for 314 yards and four touchdowns in 11 games last season for the Trojans.
"This was a hard decision, one I thought over long and hard," Farmer said. "I received input from my family and the coaches, and ultimately it was my decision. I feel like l am ready for the next level, so I am forgoing my senior year and I will work hard to prepare for the NFL."
Farmer, from Gardena, California, came to USC as a five-star and No. 1 wide receiver prospect in the nation according to Rivals.com.
He missed the entire 2013 season after tearing the anterior cruciate ligament (ACL) and medial collateral ligament (MCL) in his left knee during a non-contact drill.
Farmer caught 30 passes for 363 yards with four touchdowns in his career.
- Scooby Axson