The British swimmer cruised to victory in the 100-meter breaststroke with a time of 57.13 seconds Sunday night, shattering the mark of 57.55 he set a day earlier in the preliminaries.
Peaty blew away Cameron van der Burgh of South Africa, the defending Olympic champion who took silver this time in 58.69. The bronze went to Cody Miller of the United States, whose time of 58.87 held off teammate Kevin Cordes.
Peaty pounded the water when he saw his time. Then he balanced himself atop a lane rope, spread his arms wide and looked toward the roof of the Olympic Aquatics Stadium.
"It's surreal," he said. "After my race I needed to slap something and there was just the water right there. It was crazy. It's amazing and I probably won't be able to sleep tonight."
Sweden's Sarah Sjostrom also turned in a world-record performance in the 100 butterfly.
She led right from the start and touched in 55.48, knocking off the mark of 55.64 she established at last year's world championships.
Sjostrom sat on the edge of the deck, pumping her arms in the air, and then appeared to be overcome by tears as she climbed to her feet.
This was her first Olympic medal. Sjostrom finished fourth in the 100 fly at the London Games four years ago, missing out on the bronze by just 23-hundredths of a second.
"The feeling is totally crazy," Sjostrom said. "I didn't realize it was a world record. I knew I was the big favorite. I was under pressure, so I tried to focus on no disasters. Before the start I said to myself, 'It's just a pool. It's nothing. I know what to do.'"
She played the board game Yahtzee with her roommates in the afternoon, and then donned headphones while getting mentally prepared for the late-night race.
"I've never done that before," Sjostrom said. "I listened to music for the first time before a race. Why not?"
Penny Oleksiak of Canada took the silver in 56.46, edging out defending Olympic champion Dana Vollmer. The American, who had her first child 17 months ago, settled for the bronze in 56.63.
She'll take it.
"I touched the wall and thought, 'Please just let it be a medal,'" Vollmer said.