The game was tied at 71 when Posley made the buzzer-beating shot. The Rams had climbed back from a four-point deficit in the final minute to tie it on a Jordan Burgess layup with 18 seconds left. After Posley's shot went in to cement the 73-71 home victory, the fans at Reilly Center stormed the court.
Posley finished with 15 points while Dion Wright scored a team-high 19 points and grabbed eight rebounds. Guard Terry Larrier led VCU with 15 points off the bench, while JeQuan Lewis added 14.
VCU was playing without leading scorer Treveon Graham after he aggravated an ankle injury on Wednesday, as well as guard Briante Weber, who suffered a torn right ACL, MCL and meniscus last Saturday's loss to Richmond.
The loss was just the Rams' fifth of the season. They fall to 18-5 (8-2 Atlantic 10), while St. Bonaventure improves to 13-8 (6-4 Atlantic 10).