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 Parc des Princes 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.
"I don't believe in Santa Claus," Montpellier President Louis Nicollin, who did not travel to Paris, told Canal + TV when asked if his modest side could win the title.
"I'm gutted but then again I'm happy with a draw. Each time I go to an away game we lose. They've shown our club in a good light."
Defender Alex blasted PSG into an unlikely lead with a swerving free-kick after 41 minutes but the visitors hit back on the stroke of half-time 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 had 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.
Girondins Bordeaux earlier continued their improvement and increased Olympique Lyon's domestic woes when Yoan Gouffran struck just before the break to seal a 1-0 home win.
Bordeaux have struggled since winning the 2009 title but lie ninth and within touch of the European places if they build on this result and last weekend's stunning 5-4 victory at champions Lille.
Fifth-placed Lyon lost 2-1 at home to lowly Caen last weekend before the 1-0 Champions League victory over APOEL Nicosia in midweek and their inconsistency and nervousness in front of goal was again evident on the west coast.
Gouffran scored at the far post following a corner in a largely forgettable encounter, unlike last weekend at Lille.
In the other early Ligue 1 match, Saint-Etienne thumped Stade Rennes 4-0 to leapfrog the Brittany side into sixth.
Third-placed Lille won 1-0 at Lorient on Saturday while Olympique Marseille, in fourth, drew 1-1 with Valenciennes before next week's Champions League last-16 first leg against Inter Milan.
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