Smith held a three-shot lead after two rounds and was second only to young Thai star Jutanugarn entering the last round but shot six-over-par 78 to finish tied fifth.

South Korean Kim shot a closing 68 to catch overnight leader Jutanugarn, who squandered a seven-shot lead on the back nine for a 73 at Shoal Creek in Alabama.

But Jutanugarn regrouped to claim her second major championship title with a par at the 4th hole of their playoff after Kim missed her par putt.

Without a win in 222 starts on the LPGA Tour since her 2006 debut, 33-year-old Smith still had a decent chance when she started the final round four shots behind Jutanugarn, playing beside her in the final group.

But she mixed six bogeys and a double bogey with her two birdies to slip to 11 shots behind the winner.

Jutanugarn, the 2016 Women's British Open champion, led by seven shots after the front nine but a triple bogey on No.10 cut the lead to four and seemed to shake her confidence.

Jutanugarn of Thailand poses with the 2018 US Women's Open trophy after winning in the final round at Shoal Creek. PHOTO: Christian Petersen/Getty Images.

She still had a two-shot lead with two holes remaining, but closed with back-to-back bogeys to fall into the playoff.

"That back (nine) got me a lot," said Jutanugarn, who is set to rise to No.2 in the world.

"After you have seven-shot lead and end up in playoff, I had no expectation because I kind of got mad a little bit with my back nine.

"I (told myself) I'm going to make sure I do my best very best, because I feel like I didn't commit the back nine. I have the last chance to make myself proud."

Jutanugarn and Kim finished regulation at 11-under 277, four shots clear of third-placed Spaniard Carlota Ciganda (69) while American Danielle Kang (69) was a shot further back fourth.

Smith tied with American star Lexi Thompson (70), Taiwan's Wei-Ling Su (72) and Thai amateur Patty Tavatanakit (71).

Su Oh (73) was the next-best Australian in a tie for 17th while Minjee Lee (71) tied 34th.