Peaty - who missed last year’s event after stepping away from the sport to prioritise his mental health - had collected a bronze from the 100m breaststroke final on Monday.
After safely navigating Tuesday’s morning heats over the shorter distance, the 29-year-old three-time Olympic champion was back in action for the semi-finals.
Peaty finished runner-up in the opening race to Nicolo Martinenghi with a time of 26.85 seconds to qualify in fourth spot overall.
In the men’s 200m freestyle final, Olympic silver medallist Duncan Scott finished sixth. Hwang Sun-woo took gold for South Korea in one minute and 44.75 seconds, ahead of Lithuanian Danas Rapsys (1min 45.05secs) and America’s Luke Hobson (1:45.26).
There was also British interest in the women’s 100m backstroke final, with Lauren Cox finishing fifth in a personal best time of 59.60secs, and Kathleen Dawson touching for sixth spot.
American 19-year-old Claire Curzan (58.29) won gold, with Australian Iona Anderson, 18, in second (59.12) and Ingrid Wilm (59.18) claiming bronze for Canada.
Max Litchfield finished sixth in the first semi-final of the men’s 200m butterfly, placing 12th overall, and earlier Lucy Hope was 17th in the 200m freestyle heats.
Mona McSharry had been hoping to win Ireland’s first medal at the event, but had to settle for fifth place in the women’s 100m breaststroke final.