At the 2017 World Aquatics Championships, Peaty retained his 100 metre breaststroke title. After easily winning his heat and semi-final races, he won in the final winning the race with a championship record of 57.47 seconds. Peaty also broke his own world record twice in the 50 metre breaststroke. He recorded 26.10 seconds in the heats, and in the semi-final, he became to first man to break 26 seconds and won in 25.95 seconds. He successfully defended his 50 metre breaststroke title with another sub-26 time of 25.99 seconds in the final, completing another double at the World Championships with van der Burgh taking bronze. He won a further silver in the 4 × 100 metre medley relay at the World Championship, setting a new British record, with the same Olympic line-up of Walker-Hebborn, Guy and Scott, but missed out on a medal in the 4 x 100 metre mixed medley relay with Davies, Guy and O'Connor despite setting a new European record.