Smithpeters, who hit 3 of 4 treys, finished with 11 points and Desmar Jackson had 18 for the Salukis (13-18, 9-9 Missouri Valley Conference).
Daishon Knight had 15 points and Bobby Hunter 13 for the Redbirds (16-14, 9-9), who did not get off a potential game-winning shot.
The game was tight throughout. Illinois State had an eight-point lead in the first half and was up 36-33 at the break. Southern Illinois was up 61-54 with 5:35 left after a Smithpeters 3, but the Redbirds scored the next six.
A layup by Reggie Lynch, Daishon Knight's jumper and Nick Zeisloft's free throw gave Illinois State a 65-63 lead with 29 seconds left. But Zeisloft missed the second free throw to set up Smithpeters' heroics.