Rio Olympics 2016: Adam Peaty wins GB's first medal with swimming gold
The 21-year-old from Uttoxeter broke his own world record by winning in 57.13 seconds, well clear of the field.
Peaty is the first British man to win an Olympic swimming gold medal since Adrian Moorhouse, who won the same event in Seoul in 1988.
"It's so surreal to get Team GB's first gold," said Peaty, who finished more than 1.5 seconds clear of his nearest rival.
"I came out tonight and took the first 50 easy and came back with everything I have got.
"I did it for my country and that means so much for me."
London 2012 champion Cameron van der Burgh of South Africa took silver, while Cody Miller won bronze for the USA.
Peaty, who was already the world, European and Commonwealth champion, emulates the achievement of fellow British men David Wilkie (Montreal 1976), Duncan Goodhew (Moscow 1980) and Adrian Moorhouse (Seoul 1988) in winning breaststroke gold.
It took five days for Britain to win their first gold at London 2012 but Peaty's medal came on the second day of action in Rio.