Zlatan calls for PSG to 'wake up' after defeat
Goals from Lucas Moura and Adrien Rabiot in the first 20 minutes looked to have put PSG on course for their first win in three Ligue 1 games.
But Bastia mounted a stunning fightback as Ryad Boudebouz and Francois Modesto brought them level at half-time and Julian Palmieri's second-half double stunned the champions.
Laurent Blanc's side are now down in fourth place after Saint-Etienne won 2-1 at Reims to go third and striker Ibrahimovic called for a response to the Parisian club's latest setback.
He said: "I think we did good the first 20 minutes and after something happened that never happened to me before; winning a game 2-0 and losing 4-2 is the first time. Something happened and we have to wake up.
"We have to analyse the game, I think many factors, many small details made the difference."
PSG face a trip to Saint-Etienne for a Coupe de la Ligue clash on Tuesday and host struggling Evian in Ligue 1 next Sunday.