Ryu, who entered the day in a four-way tie for first, left the field behind thanks to six straight birdies from the ninth, ending at 20-under-par 264.
It was the second LPGA Tour win for the 2011 U.S. Women's Open champion.
Claiming second place at minus-13 was Angela Stanford, who shot a 5-under 66 at Highland Meadows Golf Club.
Sharing third at 12-under 272 were Inbee Park and Chella Choi, both of whom shot 2-under 69s.
Third-round co-leader I.K. Kim put up an even-par 71 to tie Jennie Lee (67) for fifth at minus-11.