La Salle, the tournament's 10th seed, faces second-seeded Temple (24-6) at 6:30 p.m. Friday in the quarterfinals at Boardwalk Hall.
Perhaps it was only fitting that Weingarten, who was 6 for 26 on three-pointers this season, played the role of hero.
"He takes a zillion shots a day," coach John Giannini said. "He hasn't shot it well this year, but he's one of those guys you have confidence in because he works on it so hard."
Giannini designed the play specifically for the 6-foot-8 Weingarten, who was in the game only because starter Aaric Murray fouled out in the second overtime.
"Coach G really gave me the confidence to shoot that shot," Weingarten said.
The seventh-seeded Bonnies (16-14) still had a chance to win. But Ogo Adegboye and Andrew Nicholson each missed jump shots in the final 14 seconds, and Da'Quan Cook failed on two free throws with three seconds left after being fouled on an offensive rebound.
After La Salle fell behind by 71-67 in the second overtime, Michael Davenport drained two free throws with 42 seconds left, setting the stage for Weingarten, who connected from the top of the key after taking a pass from Tyreek Duren.
"It's an amazing story," Giannini said of Weingarten, who came to the Explorers in 2007 after one season at Division III Connecticut College. "I swore to him that chances of him ever playing or getting a scholarship were very slim."
The Explorers wouldn't have been in the second overtime without two huge shots from Ruben Guillandeaux. The guard swooped in for a layup in the closing minute of regulation to send it into overtime and banked in a long three-pointer over the tight defense of Davenport with 14 seconds lift in the first overtime to tie the score.
"He's probably the best defender I've played against all year. He made everything very difficult," said Guillandeaux, who scored 18 points. "I just tried to get some space and I just threw it up there and it went in."
La Salle erased a 64-57 deficit with 46 seconds left in the initial overtime as Duren (14 points, five assists) scored off a steal and Jerrell Williams (19 points, seven rebounds added a three-point play off another St. Bonaventure turnover.
The Explorers even had two shots to win after Guillandeaux's three-pointer. Cole Stefan missed a short jumper after recovering an errant inbounds pass, and Duren, who collected the loose ball, couldn't connect on a drive with two seconds left.
The Bonnies took a 39-28 lead into halftime behind Nicholson (30 points, 13 rebounds).
"Our defense kept us in the game in the second half," Giannini said.
Next up is Temple, which swept La Salle this season with 71-67 and 90-82 victories.
"We have another opportunity to play Temple on a neutral court," Guillandeaux said. "We know them very well. We're just excited going forward."