It was the first professional victory for the 22-year-old Ernst, who shot a 5-under 67 in the final round to get to 14 under at Columbia Edgewater. Kim, the first- and second-round leader, carded a 68.
Kim missed an eight-foot par putt in the playoff that handed the win to the American.
A year ago as a rookie in Portland, Ernst shot a third-round 62 on the way to a ninth-place finish. In Ernst's mind, Columbia Edgewater was a likely place for her first career win.
''A lot of good memories coming into this year. I was very confident coming in, knowing I could make a lot of birdies,'' Ernst said. ''I knew that I didn't have to do anything special today because I had three really good rounds to start with. Coming down the stretch, I was very proud of how I handled everything.''
Ernst pulled into contention with a 5-under 31 on the front nine, highlighted by a chip-in eagle at the par-5 fifth. Kim made four birdies and no bogeys.
South Korean's So Yeon Ryu and Chella Choi tied for third at 12 under. Ryu, last week's Canadian Women's Open winner, shot 70, while Choi had a 68.
Ernst was in control after she converted a long birdie putt at the par-4 14th to get to 16 under and a two-stroke lead. But she stumbled down the stretch, missing the green at Nos. 17 and 18, resulting in bogeys.
That opened the door for Kim and Ryu, but only Kim capitalized. At the par-4 18th, Kim missed the green long with her second shot, but chipped to three feet and made the par putt to force a playoff. Ryu, at 14 under heading into 18, made double bogey after hitting her second shot from a fairway bunker into a greenside pond.
Ernst finished about 20 minutes before Kim and Ryu. First thing Ernst did, after signing her card, was call her father Mark.
''Those are probably two of the hardest holes on the course. When I talked to my dad, he said if you made those bogeys anywhere else in the round, no one would say anything different. So, really solid round,'' Ernst said.
In the playoff, Ernst rolled a 35-foot putt to within 18 inches. Kim missed the green with her second shot, chipped to within eight feet, and then missed the par putt.
Kim, a three-time LPGA winner, was bidding for her first victory since 2010. After playing the entire final round without a bogey, Kim made one during the playoff.
''I've been missing quite a few of those. I think it's just the speed of the green,'' Kim said. ''I played in a lot of playoffs, but I haven't won one yet. So it was in the back of my mind. But she deserves it.''
At 18, Ryu was in position to win the tournament outright or earn a spot in the playoff, but her tee shot landed in a bunker. It got worse when Ryu hit her approach into the water.
''These days I have been struggling with hitting fairway bunker shots. As you saw, my result was really bad,'' Ryu said. ''It feels great to be in contention again, and I saw a lot of positive things this week, so honestly, I'm disappointed with my finish, but still happy with my result.''
The three third-round leaders faded. South Korea's Mi Jung Hur shot 73 to finish at 10 under, while defending champion Suzann Pettersen of Norway and Spain's Carlota Ciganda were at 9 under after 74s.
France's Karine Icher shot the day's low round of 66 to join a group at 11 under that included Denmark's Line Vedel and South Korea's Na Yeon Choi and Eun Hee Ji. Choi shot a 68, while Vedel and Ji each had a 70.
Anna Nordqvist of Sweden was at 8 under after carding a 69. Juli Inkster, the 1999 champion, shot 72 to finish 6 under.
Gigi Stoll, a senior at Beaverton High School who won the Portland Classic Amateur Open to earn a berth in the field, shot her third 71 of the tournament to finish at 1 under.
South Korea's Jeong Jang and Hee Won Han played in their final LPGA event after announcing their retirement early last week. Jang was even par after a 72, while Han finished at 3 over following a 72.
Jang was a three-time runner-up at Portland during her career.
''Good to retire here. I'm never going to forget Portland,'' Jang said.
The LPGA Tour takes a one-week break before the season's fifth major, the Evian Championship in France from Sept. 11-14. Pettersen is the defending champion.
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