The two-time major winner took advantage of favourable conditions on Thursday at Bethesda, Maryland, to go eight under in a dazzling start to the third major of the year.
No player has ever been further ahead after 18 holes at a women's major, with Chun's nearest challengers compatriot Hye-jin Choi and Thailand's Pornanong Phatlum at three-under.
Looking to capture another Women's LPGA Championship following her 2019 success, Australian Hannah Green was in a group of nine tied for sixth at one-under that included defending champion Nelly Korda.
"I don't know what golf course In Gee is playing," Korda said after finishing her round.
Australia's world No.3 Minjee Lee, who won the US Open earlier this month, posted a 73, as did Sarah Kemp, for a share of 30th in a crowded leaderboard.
Of the other Australians, Stephanie Kyriacou was even par, Katherine Kirk was two-over, one shot ahead of Su-Hyun Oh, while Sarah Jane Smith carded a disappointing opening round of 78.
Chun's score has only been achieved three previous times in the tournament, the last occasion in 2006, but she's no stranger to going low - winning her second major title at the 2016 Evian Championship with a score of 21-under par.
The 27-year-old's nine birdies and one bogey set a new best mark for the recently remodelled Blue Course at Congressional, where Rory McIlroy won the US Open by eight shots in 2011.
Chun had a relatively modest start to her round at one-under through five holes after starting on the back nine.
She got on a roll starting at the par-four 15th hole with four consecutive birdies to make the turn at five-under 31.
A three-time winner on the LPGA Tour and US Women's Open champion in 2015, Chun had her only stumble of the day on the par-four first with a bogey.
She then fired four more birdies over the next six holes to take the commanding lead.
"The course, after a lot of rain, feels longer," Chun said.
"At the same time, greens were softer. So I think it was just a balance."