The other three athletes are Duncan Scott, James Wilby, and Luke Greenbank, who teamed up with Peaty to win the 4x100m medley relay world title in 2019.
Eight-time world champion Peaty, who became a father in September 2020, said of his selection for the Games in 2021:
“I’m very happy to make the Olympic team – it’s always a huge pride and honour to represent my country in what I do best."
British Swimming changed their selection policy after a disrupted season in 2020 due to coronavirus, and the quartet was picked by virtue of winning individual medals at the world championships in Gwangju.
The rest of the swimming team will be confirmed after the British championships in April.
Peaty's breaststroke prowess
Peaty is regarded as the greatest breaststroke swimmer of all-time, and his selection will come as a surprise to no one.
The 26-year-old became the first British male Olympic champion in the swimming pool since 1988 when he won gold in the 100m breast in Rio, while he also helped the 100m medley relay team claim silver.
Peaty has recorded the five fastest 100m breast times in history, and is the heavy favourite to defend his title in Tokyo.