Ayo Dosunmu scored 24 points, and the Illini used its swarming defense to shock the Spartans. Illinois forced 24 turnovers by Michigan State, including 17 in the first half.
Giorgi Bezhanishvili scored 16 for Illinois (8-15, 4-8 Big Ten), and Trent Frazier added 15 as the Illini handed the Spartans (18-5, 9-3) their third straight loss.
Cassius Winston led the Spartans with 21 points. Xavier Tillman added 16.
Illinois went 22 of 43 from the field (51.2 percent), while Michigan State hit 26 of 58 (44.8 percent).
Illinois led for all but a three-minute stretch late in the game, pushing ahead by as many as 14 points before Michigan State rallied to take a 64-63 lead on Kenny Goins’ 3-pointer with 6:21 remaining.
But Illinois, behind its defense, held on and took a 71-68 lead on Dosunmu’s 3-pointer with 2:35 left.
Dosunmu made another 3 with 1:57 remaining, and the Illini held on from there as the Spartans missed their last four field-goal attempts.
Illinois hosts Rutgers on Saturday.