The 6-foot-5, 260-pound Nkemdiche, 18, made the announcement during a news conference at Grayson High School. His brother, Denzel, is a sophomore linebacker for Ole Miss.
Offensive tackle Laremy Tunsil of Lake City, Fla., and wide receiver Laquon Treadwell of Crete, Ill., also committed to the Rebels Wednesday.
Cornerback Vernon Hargreaves III of Tampa, Fla., the No. 2-rated high school player in the nation, signed with Florida, which was judged to have the third-best recruiting class this season behind two-time defending national champion Alabama and Ohio State.