The 22-year-old from Toulouse, one of the poster boys of Paris 2024, reeled in reigning champion and world record holder Kristof Milak of Hungary on the final length in an extraordinary atmosphere.
The 15,000-capacity indoor arena has already been dubbed the loudest and best swimming venue in recent memory, but the volume increased to truly incredible levels as Marchand overhauled Milak in the final strokes with the crowd on its feet.
He won in an Olympic record time of one minute 51.21 seconds – the third fastest ever in the 200m butterfly.
Marchand will swim again in the 200m breaststroke final at 21:31 BST, less than two hours after touching the wall, as he attempts the rare feat of winning two Olympic golds in one swimming session.
No-one in history has completed the 200m butterfly-breaststroke double.