World record holder Leon Marchand took gold in the 400m individual medley, while Adam Peaty was defeated in the 100m breaststroke final.
Leon Marchand claimed France's first Olympic swimming gold since 2012, tearing through the field to record the second-fastest time ever in the 400m individual medley.
The 22-year-old started strongly on Sunday and never looked back, touching in 4min 02.95sec, almost six seconds ahead of Japan's Tomoyuki Matsushita in second, with American Carson Foster third.
Marchand was the overwhelming favourite after demolishing Michael Phelps' 15-year-old world record in a phenomenal time of 4:02.50 last year.
Her performance in the noisy La Defense stadium in Paris was a new Olympic record.
With reigning American champion Chase Kalisz unable to make the final and the packed crowd cheering him on, Marchand held a half-length lead after the first butterfly leg.
But it was a sensational 100-meter backstroke that left his rivals behind.
Coached by Phelps' former coach Bob Bowman, Marchand claimed a victory that made him France's first pool gold medalist since the London Games, when they won four.
At the 2021 Olympics, France only took home one medal in swimming: Florent Manaudou's silver in the 50m freestyle.
Five-time world champion Marchand will also swim the 200m individual medley and 200m butterfly as he consolidates his growing reputation.
There are no three repetitions
Italy's Nicolo Martinenghi won gold in the men's 100m breaststroke at the Paris Olympics, denying Adam Peaty a third consecutive victory.
Peaty, bidding to become only the second male swimmer after retired American great Phelps to win the same title at three consecutive Games, took silver alongside Nic Fink of the US.
His time of 59.05 was just 0.02 seconds slower than Martinenghi's.
Team USA won 1-2 in the women's 100m butterfly
Torri Huske stunned American teammate and world record holder Gretchen Walsh to claim the gold medal in the women's 100-meter butterfly on Sunday.
Walsh turned first, but Huske came back strongly to touch down in 55.59 seconds, with Walsh second in 55.63 and China's Zhang Yufei taking bronze (56.21).
Huske's victory continued a sequence in which the event had never had a repeat winner since it was first held in 1956. Reigning Canadian champion Maggie Mac Neil finished fifth.
