Besok had a game-high 23 points and also pulled down 13 rebounds as his team claimed their third victory in the competition.
Both teams are now 3-2 heading into their final group game.
Galatasay controlled the opening half and went into the break with a 33-30 lead.
James Potter's three-pointer with seven minutes to play in the third quarter put Dexia in front for the first time at 44-42.
Both teams exchanged the lead before Antonio Graves nailed a three-ball to put Galatasaray in front for good with 13 minutes to go to make it 50-47.
Dexia kept the game close, however, and reduced the arrears to 70-67 with 2:25 on the clock, but the hosts hit back with a 10-0 run to claim a vital win.
Galatasaray had four players finish in double figures.
The Turkish outfit was superior shooting the ball on the night, making 20 of 38 (52.6%) inside the arc while the visitors were eight of 20 (40%).
Moreover, they committed just seven turnovers while forcing Dexia into 15.
Potter had 14 points and 10 rebounds in defeat.
Huseyin Besok (Galatasaray Cafe Crown Player):
Dexia- Mons is a really different and good team. They are first in their league and that explains all. It was a very hard game for us, we won but it is not enough. We [also] need to win our next game against Khimik."
Chris Finch (Dexia-Mons Hainaut Coach):
"We missed a lot of shots from the free throw line. We won the first game but nobody can say that we are 26 points better team than they are. It took us a while to get in the style we play tonight. Our style is playing fast and wide open. And if you make unprepared shots, you can never establish anything in basketball. Today we didnʼt do anything to control the game."
James Potter (Dexia-Mons Hainaut Player):
"It was a very close game. But in the fourth quarter they just made the bigger shots and the free throws. We missed a lot of crucial shots. They played very well and deserved the victory tonight."