Chun's 64 equals the record for the lowest opening-round score at the Women's PGA and is a new course record at Congressional Country Club.
Later, Women's PGA Championship debutant Hye-Jin Choi shot a 69 to sit five shots behind compatriot Chun.
Defending champion Nelly Korda finished the day on one under.
Chun, who is seeking a first major win in six years, shot four birdies in a row starting on the fifth hole and nine overall in her round.
"I feel so good because I always think when my name is going with something, then I feel very honoured and it makes me keep going to play golf," the 27-year-old said of her record performance.
None of the early group could match Chun's efforts, with Korda - who recently returned after a blood clot - one of six players seven shots back.
South African Paula Reto later surpassed Korda as the 32-year-old's round of 70 left her on two under.
Northern Ireland's Stephanie Meadow was among the early chasers, but three dropped shots on her final four holes meant she finished on one over.
England's Georgia Hall completed her opening round on two over, with compatriot Jodi Ewart Shadoff one shot further back.
Women's PGA Championship organisers doubled the tournament's prize money this year, with a record $9m (£7.3m) purse on offer.
The winner of the Maryland event will collect $1.35m (£1.1m).