Kylian Mbappe scored in his final Paris St-Germain home game but the French champions were beaten by Toulouse before celebrating with the Ligue 1 trophy.
In his first appearance since announcing he will leave PSG when his contract expires at the end of the season, Mbappe struck after eight minutes.
The France forward, 25, who BBC Sport understands is set to sign a five-year deal with Real Madrid, fired in after controlling a long ball over the Toulouse defence from goalkeeper Arnau Tenas.
But Toulouse drew level within five minutes through Thijs Dallinga, and went ahead through Yann Gboho's stunning finish before Frank Magri added a late third.
PSG clinched their third consecutive Ligue 1 title - and 10th in the past 11 years - in April.
But Luis Enrique's side have now lost their past three games after Borussia Dortmund won both legs of their Champions League semi-final 1-0.
PSG have away fixtures at Nice and Metz to come in the league before the French Cup final against Lyon on 25 May.
The home supporters displayed a tifo banner for Mbappe, who joined PSG from Monaco for £165.7m in August 2017.
The World Cup winner will depart as PSG's record goalscorer, having netted 256 goals in 307 appearances for the 12-time French champions.
Mbappe took his tally this season to 44 goals and 10 assists in 47 games across all competitions - but the hosts could not build on his opener.
After Dallinga's close-range finish ensured Toulouse ended the first half level, Gboho's stunning strike from the edge of the box put the visitors on course for victory and Magri completed a deserved win in the fifth minute of stoppage time.
Despite defeat, PSG were able to celebrate their league success as the supporters stayed behind to watch the players lift L'Hexagoal - the Ligue 1 trophy - in a post-match ceremony.
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