Chris Finch's team won 82-76 to improve to 3-1 in Group F while Khimik slipped to 2-2.
Elton Nesbitt poured in 19 points and Caleb Green 18 for Dexia.
Khimik were on top 70-69 with 5:17 to go after a jumper from Sasa Bratic.
Adam Hall answered with a three-point play for Dexia and then sparked the team with his defense, coming up with a steal and going in for a dunk for a 74-70 advantage.
Green then made two free throws and nailed a jumper to cap the 9-0 run and give the hosts a 78-70 lead.
Dexia held on for a six-point triumph.
Bratic had 19 points for Khimik.
Wykeen Kelly, Khimik player:
"We knew it would be very difficult to win here, especially as Mons Hainaut had already created a lot of problems for us already in the first leg. Tonight we lost 16 balls, and in such a game these are too many [lost balls] if you ask me. Moreover we were not good enough at three points (8 from 28!) while our hosts played a fast game..."
Ben Ebong, Dexia Mons Hainaut player:
"It was a struggle for survival for us, because we knew that a victory by more than five points was important in the race for qualification to the second round. Now we have to win against Zagreb to be sure to finish first. We played very hard, and the level was also very high, especially in the paint. We have to continue on this good way in Europe and in the [Belgian] championship too..."
Chris Finch, Dexia Mons Hainaut coach:
"It was very important to win tonight, because I think we could never be beaten in Khimik, like we did one month ago. In Ukraine, we were leading [our opponent] for 39 minutes, but we lost,and here the game was very close. My players were very clear in the crucial moments,and never blowed off steam when Khimik succeeded to take the leading four or five times... I'm very satisfied with my guys..."