The world No. 94 made birdie on each of her final three holes to earn the victory by a single shot from compatriot and world No. 1 Ko Jin-young and the USA’s Amy Olson.
Remarkably, this was Kim’s first appearance at the U.S. Women’s Open and only her third professional title.
The 25-year-old started the final round five shots behind leader Hinako Shibuno but eventually finished two shots clear of the Japanese, who had two bogeys on the front nine to squander her lead early on.
The final round had to be completed on Monday after rain stopped play on Sunday.
“I’m very honoured to win the 75th U.S. Women’s Open,” Kim told broadcaster Sky. “I still can’t really soak in that I’m the champion, but it feels different winning the tournament here.
“Back in Korea, the style and the environment is different, but I’m really glad and through COVID-19 we had a lot of difficulties, but glad we had the U.S. Women’s Open held in Houston.
“It was a great win. I love this golf course, love this environment. I want to actually put a tent up here and stay a few more days.”
Kim’s victory saw her scoop the $1 million winner’s check and means three of this year’s four majors were won by South Koreans.