Cheyrou netted with a powerful drive 13 minutes from time after Onyekachi Apam's own goal had cancelled out Tongo Hamed Doumbia's opener for the home side.
The win lifted Marseille, who have now won their last seven games in all competitions, to fifth place with 37 points from 21 matches, nine points behind PSG after the leaders earned a 1-0 win at Stade Brest on Saturday.
"We struggled in the first half but we managed to turn the situation around and that's what matters," Marseille midfielder Morgan Amalfitano told French TV channel Canal Plus.
"We improved collectively after the break, there was more movement, and when we do that it's much easier."
One day after the top four teams had claimed three points, Marseille also delivered the goods after recovering from a mediocre start at the Stade de la Route de Lorient.
Doumbia opened the scoring in the 14th minute with a low 25-metre drive as Rennes were rewarded for their solid start.
Marseille, however, looked dangerous on the break and levelled one minute before half-time when a Loic Remy cross from the left was deflected into his own net by Apam.
The visitors stepped up a gear after the interval, knocking the ball around nicely although they failed to unsettle a resolute home defence.
The pressure finally paid off though when Rennes goalkeeper Abdoulaye Diallo was beaten by Cheyrou's strike from just outside the box.
Rennes dropped to sixth on 35 points, four points behind third-placed Lille, who occupy the last Champions League qualifying spot after their 3-0 home victory against Saint-Etienne on Saturday.
Earlier, Girondins Bordeaux were held to a 0-0 draw at Evian Thonon Gaillard while AC Ajaccio moved out of the relegation zone courtesy of a 2-1 win at fellow strugglers Valenciennes.
