She finished at 21-under 263, while American Lexi Thompson (66) claimed second place on 17-under.
Overnight leader Nelly Korda faded with a 74 to tie for eighth, nine strokes off the pace.
In notching her second victory of the season and sixth of her career, Kim joined countrywoman Ryu So-yeon as the second multiple winner of the LPGA Tour this year.
"I really didn't expect anything," said Kim, who started the day two strokes behind Korda.
"Maybe I think that's why I played really well today. (There were) really good names are on top of the leaderboard, and I just wanted to go out and make the most out of it, and today I was able to do that."
Thompson also had a fast start with four birdies on the front side, but stalled with bogeys at the 10th and 12th holes.
She birdied three of the final four holes to notch her fifth runner-up finish of the season.
"I'm very happy with just the way I played, the way I handled myself out on the golf course," said Thompson.
"You know, it's not my 100 percent ball-striking week, but I managed to do what I had to with how I was playing and how I was hitting the ball. Seventeen under, I'm never going to complain with that."
(Editing by Andrew Both)
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