PORTLAND, September 1, 2014 (AFP) - Ernst's victory was her first on the LPGA Tour and she also prevented a podium sweep as three of the top four on the leaderboard were from South Korea.
"Oh my gosh," Ernst said. "Unbelievable. I am so happy."
The 22-year-old Ernst closed with a five-under 67. She ended at 14-under-par 274. Kim posted a four-under 68 to join Ernst at minus-14.
Ernst, who is in her second year on the Tour, said with this victory she no longer feels like an outsider.
"Definitely when you get a win you feel like you belong," she said. "With a win you get more confidence. I am looking forward to the rest of the year."
Ernst's previous best finish came at this event last year when she shot a 62 in the third round.
"I just have good memories here," she said.
Kim and Ernst returned to the 18th hole at Columbia Edgewater Country Club for the playoff. Both found the fairway off the tee. Kim's approach stopped in the right fringe before Ernst landed her shot on the green.
Kim played her third shot to about six feet. Ernst saw her birdie try roll past the left edge of the cup.
Ernst easily knocked in her par putt and then waited for Kim. But the three-time LPGA Tour winner missed her par attempt to give Ernst her first title.
South Korean golfers Chella Choi (68) and Ryu So-Yeon (70) shared third place at 12-under.
France's Karine Icher fired a six-under 66 on Sunday. She ended alongside South Korea's Choi Na-Yeon (68) and Ji Eun-Hee (70) at 11-under 277. They were joined by Line Vedel (70). Five of the top eight were from South Korea.
Former winner Hur Mi-Jung, of South Korea, shared the third-round lead with two-time winner Suzann Pettersen and Carlota Ciganda. Hur stumbled to a one-over 73 to slide into a tie for ninth at 10-under 278, while Pettersen and Ciganda were one stroke behind at minus-nine after shooting 74s in the final round.
Leading scores on Sunday after the fourth round of the LPGA Tour's Portland Classic x-won in a playoff (par 72, USA unless noted):
274 - x-Austin Ernst 69-69-69-67, I.K. Kim (KOR) 65-67-74-68
276 - So Yeon Ryu (KOR) 70-66-70-70, Chella Choi (KOR) 68-70-70-68
277 - Karine Icher (FRA) 68-71-72-66, Na Yeon Choi (KOR) 70-69-70-68, Eun-Hee Ji (KOR) 71-69-67-70, Line Vedel (DEN) 69-69-69-70
278 - Laura Diaz 67-68-72-71, Mariajo Uribe (COL) 69-72-66-71, Mi Jung Hur (KOR) 70-65-70-73, Ilhee Lee (KOR) 71-72-67-68, Lizette Salas 69-72-68-69, Xi Yu Lin (CHN) 68-68-70-72, Paula Reto (RSA) 67-69-74-68
279 - Suzann Pettersen (NOR) 71-67-67-74, Carlota Ciganda (ESP) 70-65-70-74, Tiffany Joh 71-67-72-69