On an action-packed evening in a pumping La Defense Arena pool, China's Pan Zhanle smashed his own 100m freestyle world record while Swedish veteran Sarah Sjoestroem won the women's title.
American great Ledecky again set new standards, cementing her reputation as the greatest distance swimmer the sport has known.
She flew to the wall in a new Olympic record 15mins 30.02secs to defend her 1500 freestyle title, more than 10 seconds clear of second-placed Anastasiia Kirpichnikova of France.
Pan was also on fire, clocking 46.40 for the first swimming world record of the Games.
But the night belonged to Marchand who dethroned two defending champions to complete a historic feat.
He produced a sensational final lap to upset Hungarian defending champion and world record holder Kristof Milak to win the 200m butterfly, then suited up again to claim gold in the 200m breaststroke, roared on by a deafening home crowd.
No swimmer has ever won both at the same Olympics.
Marchand now has three gold after his 400m medley victory on Sunday -- France's first pool win in 12 years -- with the chance of another in the 200m medley.
Milak looked in control, but with the fans urging him on, Marchand made a dramatic late surge to get his hand in the wall first in an Olympic record 1:51.21, sending fans into a frenzy.
Milak was second, 0.54 behind, with Canada's Ilya Kharun earning bronze.
The 22-year-old returned to dominate the breaststroke, touching in 2:05.85, an Olympic record and the second quickest ever behind China's Qin Haiyang who failed to make the final.
Australia's Zac Stubblety-Cook was second and Dutchman Caspar Corbeau third.
An honour
Ledecky's result was never in doubt -- she has set the last six world records and now owns the fastest 20 times ever swum.
It was her eighth Olympic gold, and 12th medal overall, with the 27-year-old becoming the first woman swimmer to win titles at four Olympics after grabbing her first at London in 2012.
Among men, only fellow Americans Michael Phelps and Ryan Lochte have achieved the feat.
"I just wanted to swim a time that I could be really happy with and that was one," she said.
"I try not to think about history much. I know those names, those people that I'm up with. It's an honour just to be named among them."
Ledecky, who took bronze in the 400m freestyle, will also dive in for the 800m this week as she guns for a fourth straight title.
Should she win, she will equal the most Olympic golds -- nine -- ever won by a woman athlete, matching former Soviet-era artistic gymnast Larisa Latynina.
Pan stole Australian veteran Kyle Chalmers' thunder in the 100m freestyle, smashing the record he set at the world championships in Doha this year.
Chalmers, who won gold in Rio and silver in Tokyo, came second with former world record holder David Popovici of Romania third.
Sjoestroem was a surprise winner of the women's 100m freestyle, touching in 52.16 ahead of the United States' Torri Huske and Hong Kong's Siobhan Haughey.
Australia's Mollie O'Callaghan, the 200m champion, came fourth.
While the Swedish veteran holds the world record, she had never won gold in the 100m at a major meet until now despite owning four Olympic medals and 25 at world championships.
"I just felt like I had a really good race and I was so in my zone," she said. "This is unbelievable. I didn't think I would swim the 100 free, honestly."
