Tim Frazier hit a floater in the lane with 8 seconds left and Jermaine Marshall blocked Sam Maniscalco's layup at the buzzer to give Penn State a 54-52 upset of No. 22 Illinois on Thursday night.
Frazier finished with 12 points and nine assists for the Nittany Lions (10-10, 2-5 Big Ten), who ended a three-game losing skid.
Off a timeout, Frazier drove at the top of the key before Jon Graham's pick freed him from hounding defender Brandon Paul.
The Illini (15-4, 4-2) lost for the first time since re-entering the AP Top 25 this week. They had a short stay atop the Big Ten as the league's lone one-loss team in conference play.
Paul had 20 points to pace the Illini, while Meyers Leonard added 15.
The Illini chipped away at a nine-point deficit several times in the second half thanks in part to better ball movement. Paul's 3 from the left wing with 39 seconds left tied the game at 52.
But Illinois couldn't break through after shooting 33 percent in the first half and going scoreless for an 8-minute stretch spanning both periods. Illinois shot 34 percent (17 of 49) for the game.
Illinois couldn't exploit after Frazier, Penn State's point guard, picked up his fourth foul with 5:12 left and the Illini down a point.
Cammeron Woodyard finished with 17 points for Penn State, while Marshall finished with seven points and five rebounds.
But the highlight of Marshall's night was his lunging tip to deflect Maniscalco's shot at the end of the game.
Illinois' poor start from the field warmed Nittany Lions fans hoping to witness another tight game with the Illini on a chilly, snowy night in Happy Valley. Five of the last eight games in the series have been decided by two points or less. Penn State won 57-55 at home last season, over then-No. 16 Illinois.
They pulled out another close one, this time to get first-year coach Patrick Chambers his first victory over a ranked opponent in Happy Valley.
Wearing an alternate, all gray uniform for the first time in program history, the Nittany Lions scored another key win over Illinois to rock the Jordan Center. Paul scored nine points in the first half, when Illinois shot just 8 of 24 from the field to trail 25-23.
The Nittany Lions played without starting forward Billy Oliver (flu), the team's fourth-leading scorer at 7.3 points per game.