Marchand, nicknamed the 'French Michael Phelps', carried the expectations of a nation into his first event but shrugged off the pressure to touch home first in four minutes and 2.95 seconds.
Japan's Tomoyuki Matsushita took the silver medal, with Carson Foster taking the bronze.
Prepared by Bob Bowman, the coach who turned American Phelps into a gold medal machine, Marchand will bid for another three individual events as the face of his home Games.
"It was crazy. I think I will remember this one for a very long time," said Marchand.
"The emotions are very difficult to describe. It was incredible for a swimmer.
"I opened my eyes, listened to everything that was happening around. And that really pushed me to already have a great race and have a lot of fun.
"I opened my present this afternoon, this evening."
On the strength of his first on Sunday, with nerves shaken off and steely execution, Marchand could emerge with four individual golds at a single Games, a Phelps-esque achievement if ever there was one.
The American great himself was a prominent presence at the venue, where he is working with US broadcaster NBC, and he watched on as his would-be successor was raucously feted by the home fans.
In terraces festooned with tricolore flags, the crowd joined Marchand in belting out "La Marseillaise", the French national anthem, as he stood atop the podium.
