Belhanda struck after three minutes when he fired the ball past keeper Ali Ahamada from outside the box.
Olivier Giroud, the league's top scorer with 20 goals, then missed a penalty for the visitors in the 58th minute.
"We produced a beautiful performance tonight even if we could have secured victory earlier," Giroud told the Foot+ television channel.
"This win is worth its weight in gold because all the pressure is now on PSG."
The one downside for Montpellier was that they missed a host of chances against seventh-placed Toulouse.
Morocco playmaker Belhanda, one of the players of the season in Ligue 1, was once again in top form.
Toulouse created their one clear-cut opportunity in the 30th minute when keeper Geoffrey Jourdren parried Pavel Ninkov's powerful shot and Brazilian defender Hilton kicked Franck Tabanou's rebound off the line.
Giroud followed up his penalty miss by wasting another chance five minutes later after a pass from John Utaka set him up for a one-on-one with keeper Ahamada.
Toulouse have 53 points, three adrift of fourth-placed Lyon who occupy the Europa League qualifying berth.
Mathieu Valbuena pulled a goal back for the visitors, but Marseille could not muster a late equaliser.