Remy Cabella's first-half goal was cancelled out by a Mathieu Debuchy own goal in stoppage time.
Presnel Kimpembe hampered his team's chances when he was sent off shortly before the break.
The result left leaders PSG with 84 points with six games left, 17 points ahead of second-placed Monaco who host Nantes on Saturday.
Regardless of Monaco's result on Saturday, PSG will clinch the title if they beat the principality team at home next weekend.
They would have clinched the title if they had won on Friday and Monaco were beaten on Saturday.
Against a side who showed no fear, Unai Emery's side, who were without the injured midfielder Marco Verratti, barely threatened as they failed to shoot on target during the opening half.
St Etienne went ahead when Cabella poked the ball home after Debuchy’s cross was deflected into his path in the 17th minute.
PSG failed to react and the hosts were awarded a penalty after Kimpembe brought down Romain Hamouma in the box on 29 minutes.
Keeper Alphonse Areola, however, dived onto his right to deny Cabella to keep his team afloat.
Defender Kimpembe was then sent off in the 41st minute when he picked up his second yellow card for a rough tackle on Hamouma.
Emery replaced midfielder Lassana Diarra with centre back Thiago Silva and the visitors hung on in the second half.
They snatched a flattering draw two minutes into injury time when Kylian Mbappe's header was deflected into his own net by Debuchy.
(Reporting by Julien Pretot; Editing by Pritha Sarkar)
