Ariya Jutanugarn saw a seven-shot lead evaporate on the back nine but beat Hyo Joo Kim at the fourth extra hole to win the US Women’s Open in Alabama.
World number five Jutanugarn birdied five of the first nine but four shots went in three holes in a one-over 73.
Korea’s Kim, 22, carded five birdies in a bogey-free 67 to also end 11 under.
Both were level after a two-hole play-off and both parred the first sudden death hole before Jutanugarn, 22, won her second major with another par.
Jutanugarn became the first Thai major champion when she won the 2016 Women’s British Open and was four shots ahead overnight at Shoal Creek.
She made her fifth birdie of the day at the ninth but drove into the trees and ran up a triple bogey at the 10th, before dropping another shot when she failed to find the green in regulation at the 12th.
And when Kim, the 2014 Evian Championship winner, holed a putt from off the green at the 15th, for her fifth birdie of the day, the lead was reduced to a single stroke.
But after a long wait on the tee at the par-three 16th which she spent calmly sitting against her golf bag, Jutanugarn hit a majestic shot to three feet for another birdie.
When world number 67 Kim, playing in the penultimate group, missed a five-foot birdie chance at the 17th it appeared to hand the Championship to Jutanugarn.
But the Thai went through the green at the par-five 17th, duffed a chip and had to hole from six feet to only drop one shot.
Then at the last another shot went after she put her second shot into a greenside bunker and failed to get up and down in two.
The two-hole play-off began at the 14th and Kim holed another long putt from the edge of the green for birdie, but she found a bunker at the 18th to drop a shot, while Jutanugarn sank a four-foot putt to take the Championship to sudden death.
The players went back to the 14th, which they both parred.
That meant another playing of the 18th and both found a greenside bunker but Jutanugarn played a sublime escape to two feet while Kim’s pulled up 18 feet from the hole and she could not hole her par putt.
They had finished four shots clear of Spain’s Carlota Ciganda in third, while world number one Inbee Park was ninth after four bogeys in a 73 and a one under total.
England’s Charley Hull was the highest placed British player, the world number 25 holed four birdies in a closing 71 for a share of 10th, 11 shots adrift.
Sweden’s Pernilla Lindberg, who beat Park in a play-off to win the first women’s major of the year, the ANA Inspiration in April, finished a distant 15 over.