A double from Marquinhos on either side of a goal from Angel Di Maria helped the Parisians cancel out Denis Bouanga's first-half opener for the hosts.
To make things worse, Timothee Kolodziejczak was also sent off towards the end of the opening half for a foul on Kylian Mbappe.
Bad weather conditions during the second period made life difficult for both sides, with Neymar going off late on with a horrific-looking injury.
Lionel Messi was man-of-the-match with a hat-trick of assists as PSG tightened their grip at the top of the table with 40 points from 15 games.
Here are the five major talking points from the match:
PSG and Saint-Etienne really went at each other in a pulsating opening stanza. The first half saw a goal disallowed, a red card, two goals and a huge miss to set the pulses racing.
Neymar thought he'd given the visitors an early lead but it was chalked off for offside. Then in the 23rd minute, Bouanga put Les Verts in front against the run of play.
A while later, tensions flared between the sides after Di Maria was stamped upon. The incident resulted in three players going into the book. Towards the end of the half, Kolodziejczak finally saw red.
The centre-back was sent off for bringing down Mbappe, although it seemed a bit harsh, and the resulting free-kick saw Messi assisting Marquinhos for the equalizer.
Just moments before the red card incident, Mbappe found himself on the receiving end of a pin-point cross in the six-yard area. Somehow, the Frenchman contrived to make a poor finish that the St-Etienne goalkeeper easily parried away.
Speaking of the red card, was that harsh on Kolodziejczak? He brought down Mbappe with an ill-timed challenge but the contact was soft.
There's no disputing the foul, but whether or not the St-Etienne centre-back deserved to get sent off for that sparked a debate among Ligue 1 fans, who're polarized about this.
Kolodziejczak wasn't the last defender at the time and at best should've been shown a yellow. In contrast, Alpha Sissoko absolutely steamed into Neymar early on but escaped with a yellow!
Refereeing standards in Ligue 1 are known to be poor, and such questionable calls only serve to raise more concerns about the same.