Alex salvaged a point with a late header to cancel out Gael Danic's first-half opener to put PSG on 74 points with three games left.
The Qatar-backed side played the second half with 10 men after Thiago Silva was sent off in the 42nd minute. Frustrations also spilled over after the match when sporting director Leonardo was seen bumping into the referee in the tunnel before being dragged away by club president Nasser al-Khelaifi.
Carlo Ancelotti's side lead second-placed Olympique Marseille, who beat Bastia 2-1 on Saturday, by seven points.
Last weekend, Marco Verratti and David Beckham were sent off in PSG's 1-0 win at Evian Thonon Gaillard and keeper Salvatore Sirigu was eventually handed a two-game ban for his involvement in a post-match brawl.
They saw red again on Sunday.
Gael Danic opened the scoring for Valenciennes in the 17th minute when he poked the ball home after Nicolas Douchez had parried a Vincent Aboubakar attempt into his path.
Zlatan Ibrahimovic's free-kick shaved Nicolas Penneteau's left post at the half-hour mark but PSG were too nonchalant.
Thiago Silva was shown a straight red card shortly before the interval after arguing with referee Alexandre Castro and putting his hand on him.
PSG came to life 15 minutes from time when an angled Ibrahimovic shot went just wide and Maor Melikson cleared an Alex header off his goal line.
Following a corner, Alex sent another header which this time went just under the bar.
A minute's silence was observed before kick-off in memory of the 18 who died in the Furiani disaster on May 5 1992 when a terrace collapsed before a French Cup semi-final game between Bastia and Olympique Marseille.
FINE STRIKE
Earlier, Yoann Gourcuff's return to form continued as he helped Olympique Lyon stay on track for a Champions League spot with a 3-0 win at strugglers Nancy.
Gourcuff, who has failed to impress since joining OL three years ago, netted his second goal in as many games with a fine strike on the turn after Bafetimbi Gomis had opened the scoring following the break.
He then set up Gomis for the third.
The result put third-placed Lyon on 63 points with three matches left, three points ahead of fourth-placed Nice who beat Stade Rennes 3-0 away earlier on Sunday.
The top two teams in Ligue 1 qualify directly for the Champions League group stage while the third-placed side play in the third qualifying round.
Nancy remain 17th on 35 points, one point above the relegation zone.
After picking up 10 points from their previous four games, Nancy were brought down to earth in the second half.
Gomis toyed with a defender in the box before sending a rising shot past Damien Gregorini.
Gourcuff doubled the tally with a curled shot fro
