The formal announcement came in a news release from the university, after The Oklahoman and Tulsa World reported in Thursday's editions that Warren had decided to turn pro. Warren said in the release that he has not signed with an agent but expects to do so soon.
"This was a stressful decision and one that I've been stuck in the middle on for a little while," Warren said. "I didn't like how things went this year with my injury and everything, but I also feel like I have the skills to take the next step. Some of the things I need to work on like being a better leader and being more vocal are things I can do at the next level."
Warren said he's spoken "extensively" about the decision with his mother and that he thinks it's time to enter the NBA. He said Oklahoma coach Jeff Capel "is in my corner" and has promised to help Warren make the transition.
"I'm very happy for Willie and definitely think he's ready for the next level," Capel said in a statement. "I appreciate everything he's done for our program and look forward to watching another Sooner in the NBA."
Warren was projected by some as a first-round pick following his freshman season, when he was the Big 12's freshman of the year. Warren opted to return to the Sooners, who were coming off an NCAA tournament regional final appearance keyed by national player of the year Blake Griffin, who was the No. 1 pick in last year's NBA draft.
Oklahoma's fortunes plummeted amid injuries, suspensions and team chemistry issues and the Sooners finished 13-18, their first losing season since 1980-81. They ended on a nine-game losing streak, the program's longest skid since dropping a school-record 10 in a row in 1964.
Warren led the Sooners in scoring at 16.3 points per game and averaged 3.3 rebounds and 4.1 assists. He missed 10 games, including eight because of an ankle injury. He underwent season-ending arthroscopic surgery on the ankle March 3 and said his ankle is "doing great."
"My freshman year I feel like I made some big contributions and I was somewhat satisfied," Warren said. "I feel like I could have scored more, but I understood the makeup of our team and it was a great feeling to make it all the way to the Elite Eight. My sophomore year was up and down and tough on all of us. It was a roller coaster ride, especially for me with my ankle injury and being sick.
"But if I had to make my decision all over again, I would make the same one to attend OU. It was the best experience of my life and I want to thank all the Sooner fans who were behind me."
Warren is the second Oklahoma player to announce he's turning pro. Fellow guard Tommy Mason-Griffin announced his plans to do so last month.
Another guard, Ray Willis, has said he plans to transfer.