The Ligue 1 champions, deprived of a host of key players, were well below their best and a spirited Reims side took advantage.
Kamara powerfully headed in the 29th minute and, even though PSG piled on the pressure in the second half, the visitors' defence stood firm.
Dia added a second in the fourth minute of injury time with a spectacular bicycle kick.
PSG were without the attacking qualities of Julian Draxler, Edinson Cavani, Kylian Mbappe and the on-loan Mauro Icardi – all absent through injury – while Marco Verratti was suspended.
It meant much of PSG's impetus in the final third fell to Neymar, whose knack of scoring late goals secured back-to-back 1-0 wins against Strasbourg and Lyon in Ligue 1.
22 - Reims are the first team to win against Paris at the Parc des Princes in Ligue 1 since Rennes in May 2018 (2-0). PSG were unbeaten in their last 22 home games in the top-flight (W20 D2). Exploit. pic.twitter.com/0fqVyEOMLk— OptaJean (@OptaJean) September 25, 2019
However, Reims packed midfield and swarmed around the Brazilian whenever he received the ball, stifling Neymar's impact, although he did go close with a free-kick that curled narrowly over the crossbar.
The visitors took a surprise lead just before the half-hour mark when Kamara netted his first top-flight goal.
PSG goalkeeper Keylor Navas almost kept out Kamara's header, from Marshall Munetsi's cross, with a spectacular one-handed diving save but the Reims midfielder's effort had too much power and it went in off the post.
44 - Paris have failed to score for the first time in 44 Ligue 1 games, it was the longest such-run in the history of the French top-flight. Down. pic.twitter.com/pJI3bIK2i7— OptaJean (@OptaJean) September 25, 2019
Thomas Tuchel's side dropped points in an away defeat to Rennes last month, and they only threatened sporadically in their quest to avoid a second league loss of the campaign.
PSG had not been beaten at home in league since a 2-0 reverse at the hands of Rennes in May 2018, and they went down by the same margin when Dia got on the end of Thomas Foket's cross and volleyed home in stunning fashion.