Striker Edinson Cavani scored a hat trick as Monaco coach Thierry Henry watched helplessly from the sidelines as his team crumbled.
The former France great has not won any of his six games in charge so far and Monaco is now 16 matches without a win. Monaco has won just one game in 18 in all competitions this season and is languishing in 19th place.
Only goal difference separates Monaco from last-placed Guingamp, a crushing fall from grace for a Monaco side which won the league title in 2017 after winning the last 12 games of the season.
It could have been even worse for Henry, with two further PSG goals ruled out for offside.
Monaco had the first attack of the game but then let in two goals inside the first 12 minutes, both initially disallowed but then given following a video review.
Monaco botched a routine clearance for the first after five minutes, with Neymar reacting quickly to collect the loose ball and pick out Cavani for a tap in. Cavani then slid in the second goal following a cross from the left.
Goalkeeper Diego Benaglio made three fine saves to keep the score down before VAR was used once more, as this time Julian Draxler’s goal was ruled out for offside.
Cavani’s third came in the 54th when he was well set up by midfielder Moussa Diaby and, after Kylian Mbappe was fouled, Neymar sent Benaglio the wrong way from the penalty spot and then danced in front of the travelling PSG fans.
Playing against the club which sold him for 180 million euros ($204 million), Mbappe showed remarkably quick feet to control the ball and score past Benaglio in the 78th but the goal was ruled offside.
Marseille beat Dijon 2-0 at home to finally get back to winning ways after losing the last four games.
Marseille lost twice in the league and twice in the Europa League to Lazio, tumbling out of the European competition at the group stage this week after reaching last year’s final.
After conceding 10 goals in those four games, Marseille finally kept a clean sheet this time.
Center half Adil Rami was involved in both goals. In first-half injury time, Dimitri Payet’s corner was headed on by Rami and winger Lucas Ocampos sent the ball in at the back post.
Payet’s cross led to the second goal in the 85th minute, headed in from close range by Rami following a scramble in the penalty area.
Marseille moved above Nice into sixth place.
Also Sunday, imposing striker Emiliano Sala moved to the top of the scoring charts with 11 goals as midtable Nantes drew 1-1 at Rennes.