Patty Tavatanakit managed to keep the hunters at Mission Hills in Rancho Mirage, California, and her lead after the third day's game turned into a win after the fourth.
The first major of the year was also 21-year-old Tavatanakit's first major win.
The final round went to Tavatanakit on 68 strokes, four under par and a total of 18 under par (270 strokes) gave her a winning margin of two balls down to second - Lydia Ko.
Ko, on the other hand, achieved an impressive climb from Saturday, when she went the final round of minus ten and keyed in five places.
- I had no idea, says Tavatanakit about Kos' journey in the results list.
- I only played my own game, did not look at the leaderboard at all, she continues.
Best Swedish, Pernilla Lindberg, is in shared place 14 after a round of 68 strokes, a total of 281.