Mike Caffey gave the 49ers (16-16, 10-6) a 59-55 lead with 56 seconds left.
Jaylen Bland hit a 3-pointer nine seconds later to pull Riverside (14-16, 7-9 Big West) within one but, after Caffey missed a jumper on the other end, Bland was called for an offensive foul with three seconds to play.
Caffey missed two free throws a second later, but the Highlanders didn't get a shot off.
Caffey, Long Beach State's leading scorer (16.3 per game), scored a season-low four points on 2-of-14 shooting, including 0-of-7 3-pointers.
The teams made 21 field goals each, but Riverside made just 8 of 16 free throws and committed 16 turnovers.