Already the world record holder, the 26-year-old left the competition in his wake at the Tokyo Aquatics Centre as he finished the 100m breaststroke in a time of 57.37 seconds – six-tenths clear of his rivals.
Arno Kamminga of the Netherlands took silver while Italy’s Nicolo Martinenghi won bronze. Peaty’s countryman James Wilby finished fifth.
Speaking to BBC Sport after the race Peaty – who has recorded the 16 fastest times in the event – admitted the historic swim meant the world to him.
"It is not about who is the best all year round, it is who is the best on the day. It is about who is adaptable and who wants it more,” he said.
"When it comes down to it I am not racing for a time, I am racing myself."
Thanking his family, partner Eirianedd, and their young son, he added: “This victory wasn't mine, it was the British team's and my family and friends' – I am just so relieved.”
Gold for Peaty looked like a foregone conclusion, given his prowess in the event and the fact he hasn’t lost over this distance in the best part of a decade.
After winning gold in Rio in 2016, Peaty has triumphed in the 100m breaststroke at the last three World Championships, has broken the world record on five occasions, and remains the only man to have ever completed the race in sub-57 seconds.
Peaty has the chance to add more medals to his collection when he participates in the 4x100m medley relay and possibly the mixed 4x100m medley relay, with heats for both events taking place later this week.
Elsewhere in the swimming, Scotland’s Duncan Scott is into the men’s 200m freestyle final after finishing fastest in the semi-final in 1:44:60. He is joined by Tom Dean, who finished fourth.
Scott’s compatriot Kathleen Dawson finished third in her 100m backstroke semi-final, reaching the final as the fifth-fastest but there was disappointment for Cassie Wild who could only manage 14th place while Sarah Vasey just missed out on a place in the women’s 100m breaststroke final.