St Etienne trailed to Javier Pastore and Zlatan Ibrahimovic's early goals, but the home side, who are unbeaten in 2013, snatched a point thanks to Alex's own goal and a superb effort by Francois Clerc.
David Beckham, making his first league start for PSG in the stadium where he was sent off against Argentina during the 1998 World Cup, took a couple of last-gasp free kicks but to no effect.
PSG have 58 points from 29 games and lead Olympique Lyon by five points as none of the top seven teams managed to win at the weekend.
St Etienne, who have not lost in their last 15 matches in all competitions, are fourth on 49 points, two behind Olympique Marseille.
Les Verts are one point ahead of Nice, who also missed out on the opportunity to take third place when they slumped to a 1-0 defeat at bottom side Nancy earlier on Sunday.
"We gifted them both goals. On the first one we made a big mistake and I think there was no penalty on the second one," Clerc told French TV channel Canal Plus.
"But we played some great football and we did not deserve to lose.
"Now we still need to look behind in the standings rather than in front of us," he added.
It all started very badly for Christophe Galthier's side.
Argentine Pastore outwitted Moustapha Sall to collect a through ball from Ibrahimovic before flicking it past Stephane Ruffier.
The keeper made the mistake of touching Ezequiel Lavezzi as the Argentine forward was running wide into the box.
Ibrahimovic converted the resulting penalty with a fine chip after his initial attempt had been disallowed by the referee because a player ran into the box before the Swede took the spot kick.
It was the Sweden striker's 25th league goal this season.
St Etienne benefited from a rare moment of carelessness by the PSG central defence to reduce the arrears as Pierre-Emerick Aubameyang's cross was deflected into his own net by Alex.
Aubameyang came close after the break with a sudden left-footed shot that was parried away by Salvatore Sirigu.
Brandao set up Clerc in the box in the 72nd but stepped on to Thiago Silva in the process. Referee Antony Gautier, however, did not blow his whistle, and Clerc unleashed a delightful shot into the far corner.
"What happened with the referee is very serious. How can he and his assistants not see something that was so obvious," said an angry Sirigu.
Earlier, Toulouse and Girondins Bordeaux played out a goalless draw.
"Frankly, that was bloody boring," Bordeaux coach Francis Gillot told a news conference.