Leonard, a 7-foot-1, 245-pound center, averaged 13.6 points, 8.2 rebounds and 1.9 blocks during his sophomore season.
"This was a very difficult decision because I love the University of Illinois," Leonard said in a statement. "But I feel the timing is right for me to follow my dream of playing in the NBA and having the opportunity to provide for my mom and family. I want to thank everyone in the Illini program for their support these past two years. It starts with coach (Bruce) Weber, who has always believed in me. He taught me so much about the game of basketball and helped me mature as both a player and a person.
"I will miss all my teammates; we've been through so much together and I consider them my brothers. The future is bright here and under the direction of coach (John) Groce, I know they'll be successful. Finally, I want to thank the fans for their incredible support. It was a dream come true to represent my state school and play at the Assembly Hall. I will always be an Illini."
Weber, who was fired March 9 by Illinois and is now the coach at Kansas State, said he is not surprised by Leonard's decision, based on the feedback the coach got from NBA circles.
"I've talked to Meyers all of last year about that through this year. After they got the new coach I felt I shouldn't talk to him again," Weber said Tuesday on "The Waddle & Silvy Show" on ESPN 1000. "I knew he was leaning toward it. We had a few discussions early last week about it. I talked to his family. The feedback I got was that he could go probably anywhere from (No.) 8 to 20. It's just hard to pass up.
"I think the thing with Meyers ... in the middle of our season one of the issues that we had to deal with is Meyers is an emotional young man. When some of these things happened he came in and said, 'Coach, if you're not going to be the coach, then I'm going to the NBA.' He felt that it would be really tough for him to have a new coach and deal with that."
ESPN NBA Insider and draft analyst Chad Ford has Leonard ranked No. 23 in the 2012 draft class.
"Big men rise the closer we get to the draft," Ford said. "Leonard is counting on it. He's got the size, athleticism, rebounding and shot blocking ability to succeed in the NBA. However, he's a project. He's pretty raw offensively and at times, he loses focus. But as one of the few true 7-footers in the draft, teams will be willing to gamble in the late lottery to mid first round."