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.
"My stroke just feels like it's in a really great place. It feels strong and I'm feeling even better within myself. I didn't think I'd go under a minute so to do it twice in one week just shows where I am."
Hungary's Katinka Hosszu, who may enter six events in Rio, took her second gold in two days with the women's 200 metres backstroke. The world champion won the 400m individual medley on Monday.
France's Camille Lacourt, silver medallist at the 2015 world championships and fourth at the 2012 London Olympics, took the men's 100 metres backstroke gold in 53.79 seconds.
World champion and record holder Sarah Sjoestroem of Sweden retained her title in the non-Olympic women's 50 metres butterfly in 24.99 seconds.
(Editing by Ed Osmond)
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