The South Korean charged into the lead on the opening day of the $3 million Evian Championship, the fifth and final major tournament of the 2016 season -- and was only caught right at the end by her compatriot Sung-Hyun Park.
Both carded eight-under-par 63 in France, falling just two shots shy of the course record set by 2014 winner Hyo Joo Kim.
"Before the round I was a little nervous today," Chun told the LGPA website. "It was very windy with a lot of thunderstorms, so I tried to focus more on my game."
Her preparations paid off. While Chun missed the cut and tied for 65th in her previous two appearances at Evians-les-Bains, on Thursday the world No. 7 hit 100% of greens in regulation and carded six birdies on the front nine in a bogey-free round.
A former maths prodigy who only took up golf to prove herself to doubters
, Chun is in her first season as a member of the LPGA Tour, having won playing rights by lifting her first major at the 2015 US Women's Open.
She missed the cut in her title defense in July before tying for eighth at the British Women's Open, and has had two top-10 finishes since to take her earnings for the year past $900,000.
On Thursday she played with world No. 2 Ariya Jutanugarn and 15-year-old Swedish amateur Julia Engstrom.
"They are so nice! Thanks to them for making me feel comfortable!" the 22-year-old said.
Jutanugarn, the first Thai golfer to win a major title after her July victory at Woburn in England, was tied for 15th after a 69.
Engstrom has struggled in majors since becoming the youngest winner of the British Women's Amateur title in June, missing the cut at the British and US Opens. She needs a big improvement to reach the weekend rounds at Evian after opening with 77.
Park, meanwhile, has followed up her good form from the US Open, where she led at the halfway stage and finished tied for third. A non-member of the LPGA Tour, she also had a top-10 finish in the season's opening major, March's ANA Inspiration -- where Chun was equal second.
She finished her round with birdies at 16 and 18 -- with an 18-foot putt -- to match Chun's flawless scorecard.
The two South Koreans ended the day with a one-shot lead over Olympic bronze medalist Shanshan Feng and American Annie Park, who is sixth in the Rolex rookie standings.
Defending champion Lydia Ko was tied for 28th. New Zealand's world No. 1 carded two bogeys and a birdie in her 70.