However, despite having more possession, PSG could have been pegged back midway through the first period when Romain Alessandrini's cross was met by Jean-Armel Kana-Biyik, but his header was narrowly to the right of Salvatore Sirigu's post.
Moments before the interval, Kana-Biyik went close again as his left-footed drive flashed wide of the goal.
Les Parisiens managed to find a second goal seven minutes after the break when Blaise Matuidi was brought down in the box by Cedric Hountondji.
Zlatan Ibrahimovic, who was restored to the starting lineup after sitting out PSG's 2-1 defeat to Benfica in midweek, converted the spot kick.
But Rennes were able to pull a goal back when Motta's handball on the edge of the box was punished when Alessandrini's subsequent free kick found the top corner of the net.
Edinson Cavani almost sealed the win with 13 minutes left, but saw his shot saved by Costil.
The Uruguayan forward made sure of the victory in stoppage time following a fast break and well taken effort from the edge of the area.
PSG now lead Ligue 1 by five points, but second-place Monaco can close the gap if they beat Guingamp on Saturday evening.