Commentators declared that the shy, smiling 22 yearold from Toulouse, known as "half man, half dolphin" for his powerful underwater push-offs, had "restored national pride" with his victories, 114 minutes apart, in the 200m butterfly and 200m breaststroke.
The cheering frenzy was not confined to 17,000 spectators raising the roof of the swimming arena - even though coaches said they had never heard anything like it. Applause and screams also erupted on streets from Paris to Toulouse, even on public transport.
More embarrassingly for France, other Olympics competitions including table tennis, tennis and a fencing final - had to be briefly paused as the noise of French spectators in the stands cheering Marchand while following on their phones interrupted proceedings.
Marchand is adored not just for his technique in the pool but for his personal story. Born in Toulouse, he comes from a family of Olympic swimmers - his mother, father and uncle - who initially did not want him to follow in their footsteps because they knew the stress that it entailed.
Indeed, Marchand, a computer programming student who loves video games and manga, and whose hobby is aviation as he hopes to gain a pilot's licence, has described himself as not naturally at ease in the spotlight.
He has been open about the emotional challenges of elite sport, and says mental wellbeing matters as much as technique for competitive swimming.
His pre-race relaxation breathing techniques and a determination to smilingly enjoy the race are crucial, he has said. "You have to take pleasure in it," he told Brut Media two years ago - suggesting this was not a given amid the pressure on the starting blocks.
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