It had looked like PSG would suffer their first home loss in 20 Champions League group stage games when Dries Mertens put Napoli 2-1 ahead with 13 minutes remaining.
But Argentina international Di Maria whipped a terrific finish past David Ospina in the 93rd minute to deny Carlo Ancelotti victory against his former club.
PSG came into the game having never beaten Italian opposition in Europe’s premier club competition and Lorenzo Insigne’s first-half effort gave Napoli the lead.
The French side levelled when Mario Rui diverted Thomas Meunier’s cross into his own goal but PSG continued to look suspect at the back.
Mertens restored Napoli’s lead and though Di Maria replied, Liverpool’s win over Red Star Belgrade meant PSG still slipped down to third in the group.