A victors via a shootout when the teams met in the Coupe de France final in April, Rennes looked set again to upstage its more glamorous opponent through Adrian Hunou's tidy 13th-minute finish.
Star forward Mbappe had other ideas, though, and his 57th-minute equaliser laid the platform for Angel Di Maria to complete the turnaround in splendid style, the substitute stroking in a stunning free-kick 17 minutes from full-time.
PSG has now claimed the season's first silverware for seven straight years and did so on this occasion without Barcelona-linked Neymar, who watched on as he missed the match through suspension.
Thilo Kehrer headed an early chance against the crossbar and seemed to be thinking about the opportunity as he dozed off a minute later.
Hunou darted in behind the defender to steer Benjamin Bourigeaud's right-wing cross past Alphonse Areola.
PSG's start would have been worse had Juan Bernat seen red and not yellow for a two-footed lunge on James Lea Siliki.
Mbappe unleashed several fruitless first-half shots before Edinson Cavani poked wide with a poacher's effort.
The former restored eventually parity with the easiest of tap-ins, Pablo Sarabia cushioning Marquinhos's lofted pass into Mbappe's path.
Sarabia almost bundled in a second moments later and it was the Spaniard who won the foul for the 73rd-minute winner.
Di Maria took ownership of the dead ball and gave Tomas Koubek no chance with a sublime strike into the top-right corner to complete the quick-fire turnaround.
Both sides now return to France to begin the new Ligue 1 season.
Title-holder PSG gets underway at home to Nimes next Monday (AEST), the day after Rennes's visit to Montpellier.