Williams led the Trojans in receiving the past two seasons.
"It was a difficult decision, but I am blessed to have some great people around me to counsel me, including my family and Coach Carroll," Williams said in a statement.
"The NFL has been a dream of mine since I can remember and I feel that it is my time to take this step to the next level. I am proud to have earned my degree from USC, and that was an extremely important part of this decision for me and my family."
Williams was a Pac-10 first-team selection at wide receiver and punt returner and was named third team All-America. He had 70 receptions for 1,010 yards with six touchdowns this past season and was ranked sixth nationally in punt returns with a 14.2 yard average. Williams caught 58 passes for 869 yards with nine TDs as a sophomore.
Williams ranks 11th on USC's career receiving list with 128 catches.