The Swede netted his 14th league goal of the season when he converted a penalty seven minutes after the break and his Uruguayan attacking partner sealed the win in added time after Thiago Motta had put PSG ahead with a superb first-half strike.
The capital club moved up to 43 points from 18 games, with second-placed Monaco on 38 ahead of their trip to Guingamp later on Saturday. Lille are third with 36.
With Ibrahimovic and other key PSG players back after missing the 2-1 Champions League defeat at Benfica on Tuesday, they were expected to easily beat 13th-placed Rennes.
PSG's scored in the 19th minute with their first effort on target as Motta netted with a low shot from 20 metres.
The leaders continued to dominate possession and were rewarded shortly after the interval when Blaise Matuidi was fouled in the box following a fine team move, allowing Ibrahimovic to make it 2-0 from the penalty spot.
Romain Alessandrini got the score back to 2-1 with a long-range free-kick midway through the second half but the hosts failed to threaten keeper Salvatore Sirigu again and PSG secured the points with Cavani's 12th of the season in added time.
(Reporting by Gregory Blachier; Editing by Josh Reich)
