South Korea's Ryu wins Jamie Farr Classic
The 22-year-old began the round in the midst of a South Korean logjam for first place.
Four of the top six on the leaderboard at the end of Sunday's round were from South Korea.
Ryu's only prior LPGA Tour win was the 2011 US Women's Open.
"It feels surreal. I can't believe it," Ryu said. "Actually, after the US Open, I don't have any wins, so that's why I really wanted this win."
She took the lead by herself for the first time with an eight-foot birdie putt at the third hole and then set a distance from the pack with six straight birdies beginning on number nine. She finished at 20-under 264.
Angela Stanford made a long birdie putt on the final hole for 66 to finish second.
South Korean compatriots Inbee Park and Chella Choi both shot two-under 69s on Sunday and shared third place at 12-under-par 272.
South Korea's I.K. Kim, who shared the third-round lead with Ryu, Jiyai Shin and Hee Kyung Seo, managed an even-par 71 to tie Jennie Lee (67) in fifth at minus-11.
Shin shot 72 to share seventh, while Seo carded a 73 to tie Beatriz Recari in ninth at nine-under-par 275.