PSG thought they were on course for three points at the Stade de Route de Lorient as Zoumana Camara made the most a rare start to open the scoring shortly before half-time.
However, Rennes clinched a share of the spoils when Ntep, who had only just entered the action as a substitute, was credited with the equaliser when it was deemed that his low cross had not touched Abdoulaye Doucoure on its way into the back of the net.
Unbeaten PSG failed to reach the heights of their 5-0 victory against Saint-Etienne before the international break, although Zlatan Ibrahimovic - who scored a hat-trick in that fixture - did rattle the crossbar.
Camara has made just six appearances in each of the past two Ligue 1 campaigns, but grabbed the opener with a scrappy finish in the absence of injured duo David Luiz and Thiago Silva.
The 35-year-old, though, was denied the winner thanks to Ntep's effort and PSG must now regroup ahead of Wednesday's visit to Ajax in the UEFA Champions League.
PSG kept the ball well in the opening stages but far too often the visitors' decision making in the final third was left wanting.
An inability to create clear-cut chances could have proved costly for Laurent Blanc's men had Pedro Henrique shown more composure when he shot high and wide with a half-volley from the edge of the box in the 22nd minute.
Ibrahimovic was unsurprisingly proving to be PSG's biggest threat and he almost capitalised on a poor defensive header from Mexer with an acrobatic effort from close range that was well saved by Benoit Costil.
The Sweden international went even closer three minutes before the break when he watched Marco Verratti's superb long ball over his shoulder and crashed a first-time side-footed volley against the crossbar.
PSG's strong end to the half was rewarded a minute later, though, as Gregory van der Wiel provided a teasing ball across the area and Camara bundled home inside the six-yard box.
Edinson Cavani tested Costil with a 25-yard effort shortly after the restart, but Rennes shocked the visitors with the leveller in the 55th minute.
Ntep, who entered the fray to replace Henrique, showed good pace down the right and his low centre appeared to be missed by Doucoure and rolled past Salvatore Sirigu into the bottom left corner.
The forward went close to a second with just under 20 minutes remaining, racing clear of the PSG defence and forcing Sirigu into a save low to his right.
Rennes defeated PSG 2-1 at the Parc des Princes the last time the sides met in May, and the hosts arguably finished the match stronger with Ola Toivonen firing a free-kick just over.
The match petered out into a draw, though, leaving defending champions PSG still searching for a first away league victory of the campaign.
