Justin Cage had 21 points and seven rebounds for the winners.
The game was tight from beginning to end.
With the contest level at 78-78 3:44 from the end after a basket from Ventspils' Tyrone Brazelton, Christopher Hill drew a foul and made two free throws for Belgacom.
After a miss by Ventspils, Hill drew another foul and went to the line and made two more free throws for an 81-78 lead.
Belgacom stayed on top for the remainder of the contest.
Belgacom out-rebounded their Latvian opponents 41-30. Of the Belgian team's rebounds, 17 were on the offensive glass.
Mareks Jurevicus scored 22 points and grabbed seven rebounds for Ventspils who slipped to their third defeat in four EuroChallenge games.