Ariya frittered away a seven-shot lead with nine holes remaining before parring the fourth play-off hole to edge South Korea’s Kim Hyo-joo and become the first player from Thailand to lift the trophy.
“I feel great,” Ariya said. “I am really excited. I’m really honoured to join the list of winners before me.”
The 22-year-old from Bangkok parred all four play-off holes at the Shoal Creek, Alabama course for her second win of the 2018 LPGA Tour season to go with the Kingsmill Championship.
“After you have a seven-shot lead and end up with you having to go to a play-off, I had no expectations,” she said.
“If I have a play-off then I’m going to make sure I do my best every shot because I felt like I didn’t commit on the back nine. I felt like I had a last chance to make myself proud.”
She finished with a one-over par 73 and a 72-hole total of 11-under 277.
South Korea’s Kim closed with a five-under 67 to force the play-off and had two long putts to win in the extra session but couldn’t get them to fall.
“I started off quite a few strokes behind the leader, so I didn’t really think that I was going to come through,” Kim said.
Both parred the third play-off hole and they returned to the par-four 18th, where Ariya’s superb bunker shot left her with a tap in for the win.
“I have a lot of confidence in my bunker shots right now,” said Ariya, who also won the 2016 British Open.
She had appeared to be headed to victory hours earlier, after a birdie at the ninth hole moved her to 16-under and a seven-stroke lead.
Ariya’s collapse began with a triple bogey seven on number 10 and continued with a bogey at 12.
She carried the meltdown through with back-to-back bogeys on 17 and 18.
“On 10 I just didn’t feel comfortable to hit the three wood and honestly I shouldn’t have hit the three wood, but I did. After that I kinda played a little bit scared,” she said.
She righted the ship in the play-off which went to a sudden death format after the two-hole aggregate start failed to produce a winner.
Not only does Ariya become the first Thai to win a US Women’s Open title but she is the first champion from a country other than South Korea or the US since Sweden’s Annika Sorenstam in 2006.
Kim, chasing a second major title to go with her 2014 Evian Championship, had five birdies in her bogey-free round.
“I feel very good about how I played, especially that I did not have a bogey today in the final round. That, I am extremely happy about,” Kim said.
Spain’s Carlota Ciganda shot a three-under 69 to finish alone in third place, four strokes back of Kim and Ariya.
American Danielle Kang (69) was four shots back of Ciganda in solo fourth.
Thailand’s Patty Tavatanakit, Hsu Wei-Ling, of Taiwan, Sarah Jane Smith of Australia and Lexi Thompson of the US tied for fifth at two-under 286.