The 21-year-old world record holder, still in hard training and unshaven, then went back into the pool and did it again to help the hosts end the night with another gold in the non-Olympic 4x100 mixed medley relay.
While Peaty's time of 58.36 seconds was well off his 2015 record of 57.92, it was quicker than he had expected given that he was treating the event as a training exercise.
"I'm in one of the hardest training blocks but I can still produce a 58.3 or 58.6 on demand," he told Reuters. "That's a good sign for me."
Peaty will make his Olympic debut in Rio in August as Britain's hottest prospect in the pool. He does not lack confidence.
"It is going to be a good race in Rio because Britain is looking in a very strong place," he said. "I don't shiver in the pressure, I thrive in the arena so come Rio we can hopefully nail it."
Peaty was well ahead at the turn and touched out 1.37 seconds ahead of compatriot Ross Murdoch, the 2015 world silver medallist, in a British one-two. Lithuania's Giedrius Titenis took bronze in 1:00.10.
The medals represented a better haul in one race than British male swimmers managed in the same pool during the 2012 London Olympics, where they came away with only a solitary silver.
"We are going to be on top strength," Peaty said of Rio.
