Kim posted a 1-under 71 to join Soo Young Moon and Lorena Ochoa at 10-under- par 206. Moon closed with back-to-back birdies to shoot 69, while Ochoa fired a 67 to get into the playoff on the Palmer Course at Turtle Bay Resort.
All three players found the fairway off the tee at the 18th, which was the site of both playoff holes. Moon's third shot stopped 18 feet from the cup and she sank that putt.
Kim drained a 10-foot birdie effort to stay alive. Ochoa, who left herself only 8 feet, missed her birdie putt short and right of the cup, so she was out of the playoff.
'I was thinking about going for the green in two, but I had a bad lie so I decided to lay up,' Ochoa said. 'I am pleased with the way I played the final two rounds after I didn't have a good first round.'
Both Kim and Moon found the fairway again off the tee on the second extra hole. Each player laid up with their second to the par-5 and both came to rest in the right rough.
Kim's approach rolled to within 2 feet of the cup. With the pressure on, Moon knocked her third shot to 12 feet. Moon ran her birdie putt 4 feet by the hole, but made the par putt coming back.
Kim ended the extra session right there as she kicked in her 2-footer for birdie and the win.
'I did not think about winning today,' admitted Kim, who pocketed $150,000 for the win. 'I'm very happy. It seems like I am dreaming. I was trying to have fun.'
Karen Stupples matched Ochoa's 67 on Saturday. That helped Stupples to move into fourth place at 8-under-par 208. LPGA Tour rookie Morgan Pressel and U.S. Solheim Cupper Natalie Gulbis each posted 70s in the final round to share fifth place at minus-7. They were joined there by Miriam Nagl (68).
Kim carded her lone front-nine birdie on the par-5 third. She parred the other eight holes on the front side, then parred the 10th and 11th.
The 21-year-old birdied the 12th to move to 11 under. However, Kim stumbled to a bogey at the next. She parred her final five holes to end at minus-10.
Moon birdied the third and sixth to climb to 9 under. After a dropped shot on No. 8, Moon got that stroke back with a birdie at the 10th.
She traded a bogey for a birdie from the 13th and again from the 16th. Moon knocked her approach to 3 feet at the 18th to close with a birdie and get into the playoff.
Ochoa birdied the third, but gave the shot back on four. She birdied four of five holes from the sixth to jump to minus-nine. The Mexican tripped to a bogey on the 11th, but atoned for that error with a birdie on 12. She drained an 18-foot birdie try on 16 to end at 10 under.
Juli Inkster and Cristie Kerr each shot 68s to move into a tie for eighth place at 6-under-par 210. They were joined there by Johanna Head (70) as well as second-round co-leaders Becky Iverson (75) and Sung Ah Yim (75). They entered the final round tied for the top spot with Kim.
LPGA Tour Rookie of the Year Paula Creamer led a group of nine players one stroke further back at minus-5.