Richardson's buzzer-beater followed one by Hawaii's Vander Joaquim, who hit a jumper at the end of regulation to force overtime. Before that, Brandon Paul's basket with 2:08 to play had given the Illini (3-0) their first lead of the game.
The teams traded baskets in the extra session before Jace Tavita made one of two free throws to give the Warriors (3-1) a 77-75 lead with 7 seconds left and set the scene for Richardson, who finished with 16 points.
Brandon Paul scored 25 points, including five 3-pointers, for Illinois, and Tracy Abrams added 22. The Illini improved to 7-0 against Hawaii.
Joaquim led the Warriors with 22 points and 11 rebounds, and Brandon Spearman added 20 points and nine boards.