John Utaka's header looked to have sealed a famous away win for Montpellier but France striker Hoarau tapped in after a good move to send the home fans delirious.
Montpellier, top scorers in Ligue 1 this term with 47 goals, have surprised most pundits by lasting the course until February and deserved to at least stay a point behind the leaders after 24 games of the 38-match season.
Defender Alex blasted PSG into an undeserved lead with a swerving free kick after 41 minutes but the visitors hit back on the stroke of halftime through Younes Belhanda as a team who finished way down in 14th last season showed immense character.
The south-coast side made a strong start but PSG boast the top flight's best defence and managed to repel the early threat.
The defence then went on the attack when centre back Alex, who joined PSG last month alongside his former Chelsea coach Carlo Ancelotti, scored in style to make light of normal free kick taker Javier Pastore being left on the bench.
However, Morocco playmaker Belhanda soon has the PSG defence blushing as pink as the Ligue 1 matchball when he ghosted in to nod home Hilton's perfect cross.
Nigeria's Utaka thought he had won the game on 83 minutes with a diving header which goalkeeper Salvatore Sirigu should have saved but Hoarau had the last word.