Caeleb Dressel collected his eighth career Olympic gold medal, and a first of the Paris Games for the U.S. team, when he anchored them to a second successive 4x100 meter freestyle title on Saturday.
Australia took silver and Italy the bronze, with China missing out on the medals in fourth place.
Jack Alexy, Chris Guiliano, Hunter Armstrong and Dressel combined in a time of 3 minutes 9.28 seconds to beat Australia by 1.07 seconds in the last final of a raucous opening night at the La Defense Arena.
"Relays are a little more special to be honest," Dressel said. "So doing it with these guys has been awesome. It takes me back to my first gold. It really doesn't get old. Really special standing on the podium with these guys watching the flag go up. I'm extremely proud of them. It made my job easy."
An emotional Dressel pumped both fists high in the air in triumph and delight before even stepping up to accept the latest gold medal placed around his neck. He held teammate Hunter Armstrong in a prolonged embrace after Armstrong swam the fastest leg to give Dressel some wiggle room coming home, then Dressel raised his arm in rhythm to chants of "U-S-A!"
Matt King and Ryan Held swam in the morning heats but were switched out in favor of Alexy and Guiliano for the final. King and Held will also receive gold medals.
The Americans took a lap around the arena after the victory, offering high-fives and hugs.
And Dressel quickly made his way to find wife Meghan and 5-month-old son August Wilder, finally spotting them and sprinting over for a hug and kiss before taking the baby in oversized headphones into his arms.
"It's really special. Making the team in front of him and then winning a gold in front of him, just checking little boxes that I never would have thought to create throughout my career," Dressel said. "So that was a really special one tonight."
Dressel will still try to defend his golds from the Tokyo Olympics in the 100 butterfly and 50 freestyle. He also won the 100 three years ago and 4x100 medley. Dressel captured golds in the two relays at Rio de Janeiro eight years ago, too.
In the women's 4x100, Australia made it 2-for-2 against its rivals from the U.S., claiming its fourth straight Olympic title in that event.
The quartet of Mollie O'Callaghan, Shayna Jack, Emma McKeon and Meg Harris set an Olympic record with a winning time of 3:28.92.
The Americans -- Kate Douglass, Gretchen Walsh, Torri Huske and Simone Manuel -- rallied to take silver in 3:30.20. They edged China's team of Yang Junxuan, Cheng Yujie, Zhang Yufei and Wu Qingfeng by a tenth of a second.
Reuters and The Associated Press contributed to this report.