Park -- who has won both LPGA Tour majors this year -- entered the clubhouse after 54 holes with a one-stroke lead at 12-under-par 201 after birdying the final hole to complete a final round of 4-under 67.
But Ryu caught her atop the leaderboard with her own birdie on No. 18, carding a 2-under 69.
Returning to the par-five 18th hole, Park chipped from the first cut of rough to within 3 feet of the cup, putting pressure on Ryu to hole out from back fringe of the green.
She almost did, rolling her fourth shot just inches past the hole, but it left enough of an opening for Park to easily tap in for birdie to wrap up her fifth LPGA Tour win of the season and eighth of her career.
She'll seek a third straight major title at next week's U.S. Women's Open.
Claiming third at 11-under was Mika Miyazato (67).
Stacy Lewis (71), amateur Lydia Ko (68) and I.K. Kim (69) finished knotted for fourth at minus-10.
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