Leonard Williams emerged from the McKay Center on Monday looking relieved.
The two-time All-American defensive lineman had just told Coach Steve Sarkisian that he would make himself available for the 2015 NFL draft.
"It was a hard decision because I love college football but overall I have to take advantage of my opportunity and look forward to my future," he said as he pulled off a set of headphones.
Williams, a 6-foot-5, 300-pound junior, is projected as a possible top-five pick in the draft.
He wore black and white sweats to his meeting with Sarkisian.
Asked if he wanted to play for the Oakland Raiders, who have the fourth pick in the draft, he said, "I would love to play for Oakland."
Williams finished the season with 80 tackles, including 9 1/2 for losses and seven sacks.
He said he made his decision to turn pro after the Trojans' 45-42 Holiday Bowl victory over Nebraska on Saturday night. Williams had nine tackles, including a sack.
"Right after the game when my shoulder was a little sore," he said, adding, "Coming back you don't know what's promised."
Williams said he was in the process of hiring an agent.
Sarkisian, who just completed his first season as head coach, wished Williams luck.
"He was a great player here for all three of his years," Sarkisian said in a statement on the school's athletic department website announcing Williams' departure. "Just like all of our players, Leonard is a Trojan for life and we will always support him."
Tailback Javorius Allen and receiver Nelson Agholor also were scheduled to meet with Sarkisian on Monday.
Ed Orgeron, who helped recruit Williams and coached him for two seasons as a position coach and interim head coach, said Williams was fulfilling a dream.
"We recruited guys who want to be first-round picks," Orgeron, who left the program before last season's Las Vegas Bowl, said in a phone interview. "Leonard wanted to be a first-round pick. Here's his opportunity. We used to tell guys, 'If you're a top-10 pick you've got to go.
"The University of Southern California was great to Lenny and Lenny was great to the University of Southern California.
"He's one of the best players we've seen at USC… and an even better person."
In a statement on USC's website, Williams said:
"This was a very hard decision for me. I love USC, Coach Sark and the coaching staff, my teammates and the fans, and I love the game of college football. It's hard to leave all of that. I know life is never the same once you leave college.
"But I feel like it's the right time for me to go. I feel like I'm ready. I'm looking forward to the new challenge that the NFL presents."
"I had a great experience at USC and enjoyed all three of my years here. I was coached by outstanding coaches and I played with great teammates. I'm thankful to all of them, as well as to the support I got from the fans. I'm proud and happy to have been a part of the Trojan Family. I'll always be a Trojan. Now, I get to be part of the great tradition of USC alums in the NFL. I'm excited to see what's coming next for me."