The Broncs (10-15, 7-6) trailed 71-64 before going 3 of 4 from behind the arc in the final three minutes.
After Antoine Mason’s jumper gave the Purple Eagles (9-16, 4-9) a 73-71 lead with 6 seconds left, Jonathon Thompson found Penn for the win.
Rider, which trailed for the final 32 minutes, went into the locker room down 41-31 at halftime. The Broncs then made all but three of their second-half field goals from behind the arc. Rider was 11 of 18 from long range (61.1 percent) and 14 of 29 from the field (48.3 percent) in the second half.
Jeff Jones led Rider with 18 points.
Marvin Jordan scored 24 points and Mason had 15 points and 12 rebounds for Niagara.