The 20-year-old from Staffordshire added 50m breaststroke gold to his success over 100m on Monday, becoming the first man to hold both world titles simultaneously.
Peaty then returned to the pool to help the 4x100m medley team finish first and set a world record time. It was Britain’s fourth gold of the week – their best ever haul at a World Championships.
“We fought for every inch there,” he said, after winning alongside Chris Walker-Hebborn, Siobhan-Marie O’Connor and Fran Halsall. “As you can tell, we’ve got nothing left – we left it all on the battlefield.”
Peaty, who his mother says was scared of water as a child, could yet claim a fourth gold of his debut World Championships in the 200m breaststroke.
High-diver Gary Hunt started a fine day for Britain in Kazan when he won the 27m platform event at the World Championships, which is taking place in parallel to the swimming competition.