Igor Tudor's second-placed side could have cut the gap to PSG to four points with a victory ahead of the defending champions' Sunday game against Lyon.
But instead, Marseille dropped points for the third time in five league outings and could be overtaken by third-placed Lens when they visit Rennes on Saturday.
Arnaud Nordin finished off a quick counter-attack to give mid-table Montpellier a 12th-minute lead after a brilliant burst from Wahbi Khazri.
Marseille levelled shortly before half-time as Matteo Guendouzi stroked home a penalty awarded for handball.
But OM created little and Khazri went close to a winner for Montpellier midway through the second period when he was denied by home goalkeeper Ruben Blanco.
Marseille dominated possession late on but could not find a goal which would have given them renewed hope of a first league title since 2010 after PSG's shock home loss to Rennes before the international break.
