Thomas Tuchel watched from the stands as a patchwork side ravaged by injury failed to protect Dani Alves' stunning opener and fell to a second straight defeat, having been thrashed 5-1 at second-placed Lille at the weekend.
There was perhaps more embarrassment in conceding three times to a relegation-threatened team they had beaten in each of their past 14 top-flight meetings.
Diego Carlos did the bulk of the damage for Nantes with two goals either side of Majeed Waris' tap-in in the 44th minute.
And, though substitute Metehan Guclu grabbed a late consolation for PSG, their eventual celebrations of a sixth title in seven seasons look certain to be muted.
PSG needed a lift after their crushing loss to Lille and appeared to have it when wing-back Alves lashed a sweet 30-yard strike over Maxime Dupe in the 19th minute.
That joy disappeared barely three minutes later as Diego Carlos nodded in Valentin Rongier's corner, and Nantes compounded the away side's misery shortly before the break.
Kalifa Coulibaly dispossessed Thilo Kehrer and squared for Samuel Moutoussamy, whose side-footed effort was turned in by striker Waris.
The second half brought little defensive improvement for PSG as another Nantes corner evaded several players at the near post and squirmed in off Diego Carlos' standing foot.
A fourth should have followed after Gianluigi Buffon wandered well outside his area but Coulibaly, gifted the ball by Leandro Paredes, shot over an open goal.
Guclu pulled one back via Colin Dagba's cut-back, but the match will be remembered for PSG stumbling into another hurdle in their awkward attempt to cross the finish line.
What does it mean? Reinforcements needed imminently
Kylian Mbappe was the latest star to slide out of PSG's decimated squad as conflicting reports mentioned a rumoured falling out with head coach Tuchel and minor fitness concerns as possible explanations. That left the champions without each member of their star-studded front three and desperate for established leaders like Marco Verratti, Thiago Silva and Edinson Cavani to return.
Rongier finds his range
PSG's inability to defend set-pieces became obvious early in proceedings and captain Rongier ensured Nantes exploited the vulnerability. The midfielder's delivery was excellent for both of Diego Carlos' goals, and his usage of the ball in general play was of an equally high standard.
Kehrer struggles continue
Routine mistakes might be acceptable from the younger players thrust into action for PSG, but defender Kehrer is now 22 and making them on a regular basis. His latest indiscretion was to all too easily allow Coulibaly to bully him off the ball in the first half, an incident that summed up the away side's timid defending.
Key Opta Facts:
- Nantes have won their first game against PSG in Ligue 1 since October 2004 (1-0). It is also the first time they scored at least three goals against PSG in the top-flight since January 1995 (3-0).
- Nantes have won four of their last nine games in Ligue 1 (D1 L4), after winning none of the previous five (D1 L4).
- PSG have lost two consecutive games in Ligue 1 for the first time since November 2011 (also two).
- The Parisiens have lost three of their last seven away games in Ligue 1 (W4), after losing none of the previous 17 (W12 D5).
- PSG have conceded at least three goals in two consecutive Ligue 1 games for the first time since March 2009 (v Marseille and Toulouse).
- Metehan Guclu (20 years old) has scored his first goal in Ligue 1 after only 15 minutes and with his second shot.
If Lille fail to win at Toulouse on Sunday, PSG will be crowned champions a matter of hours before they welcome Monaco to Parc des Princes. Nantes also have three days' rest before hosting fellow strugglers Amiens.