The 20-year-old, who set the 50m world record of 26.42sec in Tuesday’s semi-final, clocked 26.51 in the final with South Africa’s Cameron van der Burgh, the 2013 world champion, taking silver, 0.15sec back.
Kevin Cordes of the United States earned bronze at 0.35sec.
“That was really good. I missed a couple of strokes, but I just stayed on top of the water,” said Peaty.
“It was just a race to touch the wall first and thankfully it was my day.
“I got the world record yesterday and secured the title today – I couldn’t really ask for more.”
Peaty then backed up his individual success by helping the British quartet win the 4x100m mixed medley relay in a new world record for his second gold of the night and his country’s third of the day after Gary Hunt’s victory earlier in the men’s high-diving competition.
Britain’s 3min 41.71sec was the third time the world record had been set in the event during the day after Russia, then the US lowered the mark in the morning’s heats.
Peaty, the Commonwealth champion, has made a stunning world championship debut in Kazan by matching David Wilkie’s double — the Scotsman won the 100 and 200m breaststroke titles at the 1975 world championships — and setting two world records.
He will be bidding for a third individual title in Kazan when the 200m breaststroke heats start on Thursday with the final on Friday night.
Having beaten Van der Burgh on the wall in the 100m final, Peaty again came out on top ahead of his South African rival over the sprint distance.
But Van der Burgh was delighted to pick up his fifth consecutive worlds medal in this event — only US star Michael Phelps has picked up more, with six in the 100m butterfly between 2001-11.
“It’s just nice to be consistent and always up with the best in the world. I am very honoured and privileged,” said Van der Burgh, who is relishing defending his 100m Olympic title in Rio next year.
“I think Phelps is the only other one to get five consecutive medals.
“I think I am the type of swimmer when I come into a competition I have one big swim in me and I think I always try and use it up on the 100m.
“That’s the main goal for me, the Olympic Games.”
Van der Burgh said he had sympathy for compatriot Chad le Clos after the Olympic champion had earlier lost his world crown to Hungary’s Laszlo Cseh in the men’s 200m butterfly final.
“Tonight wasn’t South AFrica’s night with Chad getting silver and me getting silver, so we’ll take silver this year and we’ll kick on next year,” said Van der Burgh.
“So it’s a lot of motivation, yeah.” – Agence France-Presse