Peaty, who set a world record on Sunday by swimming 56.88 in the semi-finals, touched home in 57.14, ahead of compatriot James Wilby (58.46) and China's Yan Zibei (58.63).
Having achieved 'Project 56' - the first man to go under 57 seconds for the 100m breaststroke - British Olympic champion Peaty was soley focused on winning gold on Monday - but he turned inside a record pace of 26.60 as he led his rivals by more than half a body.
Peaty makes yet more history at ISL grand final
Having won in 2015 and 2017, the 24-year-old is now unbeaten in five years over 100m in major competitions.
Peaty ploughed through the final 15m to seal a third world triumph - and after competing in the 50m on Tuesday, defending his Olympic title in Tokyo in just over a year will be his next target.
Meanwhile, Canada's Margaret MacNeil won the women's 100 metres butterfly world crown in Gwangju to deny Sarah Sjostrom a fifth title in the event.
The Swede was on world record course at the halfway point but was overhauled by MacNeil, who touched home in 55.83 ahead of Sjostrom (56.22) and Australia's Emma McKeon (56.61).
'My biggest demon today was going faster than yesterday', says Peaty
Perfect partnership bears fruit for Peaty and Marshall