Expectations of the home side were at fever pitch at a packed Stade de France, but the first half was somewhat of a letdown. A light Paris drizzle prompted handling errors and it ended scoreless.
French coach Jerome Daret started star man Dupont on the bench, as he has for most of the tournament, but unleashed his main playmaker early in the second half.
But it was the Blitzboks who opened the scoring, Tristan Leyds taking advantage of some lacklustre defending to score in the corner.
France hit back quickly, a darting run by Varian Pasquet setting up Rayan Rebbadj to score under the posts, a try that was crucially converted to give France the lead.
Quick thinking by Dupont set up Rebbadj for his second with seconds to spare, sending the boisterous home crowd into raptures.
An acrobatic effort from the impressive Jordan Sepho after the buzzer put the icing on the cake and sent Les Bleus into the gold medal match, scheduled for 1745 GMT.
In the second semi-final, Fiji, the undisputed kings of sevens, defeated Australia 31-7.
Fiji came into the match hoping to put a ponderous quarter-final performance against Ireland behind them.
But they too initially struggled with the wet conditions that hampered their usual free-flowing offloading game.
The Australians seized on a series of errors to scramble the first try, Ben Dowling outpacing the defence to an intelligent grubber kick and touching down.
Fiji have never lost an Olympic rugby sevens match, however, and were not about to give their proud record away easily.
They were level almost immediately, Joji Nasova breaking clear of a tackle in his own 22 and racing the entire length of the pitch to dot down under the posts.
And normal service was resumed early in the second half, a series of precise offloads releasing Iosefo Baleiwairiki for another converted try to give Fiji the lead.
Now the Fijians were cruising, a slick switch move slicing apart the Australian defence to send Kaminieli Rasaku over, effectively ending the game as a contest.
A Terio Tamani Veilawa penalty and diving try by Selestino Ravutaumada put the game further out of reach for Australia and set up the dream final for fans and organisers.
