Kylian Mbappe scored twice with Neymar and Mauro Icardi also on target for Mauricio Pochettino’s side.
Montpellier goalkeeper Jonas Omlin was sent off with the game goalless.
Omlin was shown a yellow card for bringing Mbappe down outside of the area, but that was made red after the referee viewed the pitchside monitor.
Mbappe broke the deadlock soon after with a neat touch and dinked finish from Angel Di Maria’s pass.
PSG then added three second-half goals in as many minutes.
Neymar finished a neat move, Icardi hammered home from Alessandro Florenzi’s cross and Mbappe tapped in for his second following a swift break.
PSG sit three points above second-placed Lille, having played a game more.