Laurent Blanc's PSG side were denied a goal when Edinson Cavani raced through before rounding the keeper and slotting the ball into an empty net only to be adjudged marginally offside.
Matuidi broke through in the 61st minute with his second league goal of the season, a powerful left-foot shot from a tight angle after good work by Maxwell and Ezequiel Lavezzi.
Montpellier substitute Souleymane Camara gave PSG a scare in stoppage time when his spectacular overhead kick from 12 metres was just off target.
Champions PSG are top on nine points, three ahead of Stade de Reims and Caen, the only other teams with maximum points so far, who visit Nantes and Nice respectively on Saturday.
Montpellier are bottom with no points from three matches.
(Writing by Tony Jimenez; Editing by Ken Ferris)
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