Richardson, a junior, led all DTs in the SEC with 75 tackles this season. He had 10½ tackles for loss, four sacks, forced three fumbles and blocked a kick in a standout season.
"This has been my dream ever since I was a little kid," Richardson said in a statement released by the school. "It's still not easy leaving Mizzou like this, but I know I'm ready. I really appreciate all of the support I got here at Mizzou, and I know that they will help me and my family with this next step. I'll always be a Tiger," he said.
Missouri suspended Richardson on Nov. 15 for a game against Syracuse. Richardson missed multiple classes and failed to complete the punishment given to him for missing them, sources close to the program told ESPN.com.
He also caused a stir this season by criticizing Georgia for playing "old man football."
Missouri has put a number of defensive starters into the NFL in recent years, including San Francisco's Aldon Smith, who leads the NFL in sacks this season, Atlanta's Sean Weatherspoon and William Moore, and Pittsburgh's Ziggy Hood.
"I've always said that when guys have opportunities like these, if they're ready to go, then they should go," Missouri coach Gary Pinkel said in a statement. "I have no doubt that Sheldon is ready to play in the NFL, and while we'd love to have him for another year, the important thing is that this is the right decision. It will be fun watching him play on Sundays."