Kim's approach shot in regulation barely cleared the water, but stayed in the fringe fronting the green. After Park hit a brilliant lag putt to inches for a par to stay at 11 under, Kim chipped in from 18 feet to force the playoff.
She held both hands high, stared into the sky and grinned for the first time after 5 hours of immense pressure, slapping hands with her caddie.
"I just focused on getting it to the hole because that was my only chance," Kim recalled as she was interviewed on TV - so excited that for the first time all week she spoke to the media in English.
In the playoff, staring into the setting sun at breezy Ko Olina, Kim hit an approach that barely got over the water again, then bounced twice and dropped into the hole for eagle.
"I was thinking I was between a 7- or 8-iron," Kim said. "And then I was choosing between a cut or a normal shot, so I chose 8 normal, and it was good choice."
Kim couldn't see the ball go in and neither could Park, but the roar from the crowd made it obvious. Park's attempt to match Kim's shot, and win her 14th LPGA title, came up short.
"Inbee said two things to me," Kim grinned. "How did you get that in? And congratulations."
It was an astonishingly quick ending to a draining day.
Kim closed with a 1-over 73 in wind gusting to 30 mph to match Park at 11-under 277. Park finished with a 71. I.K. Kim bogeyed the final two holes for a 74 that left her two strokes back.
The three players were tied for the lead with two holes left in regulation.
I.K. Kim, trying to win an LPGA event for the first time since 2010, bogeyed the 17th, three-putting after converting clutch putts all day. Park sank her par putt from 20 feet to get up and down from a bunker, and Sei Young Kim drained a twisting downhill 16-footer to keep pace.
"I told myself, 'This is probably the last opportunity I have to stay in this tournament,'" Sei Young Kim said.
It turned out to be the next-to-last opportunity, and when she saved par on the 18th to force the playoff, she wasted no time taking the $270,000 first prize that bumped her to No. 1 on the money list.
She's the first two-time winner on the LPGA Tour this year, after taking the Bahamas event - also in a playoff - in February in her second start as a tour member.
"Today, I didn't see it coming," Sei Young Kim said. "I don't know what I've done again to deserve this, but I feel like every time I win a tournament I have to do something crazy or overcome some type of obstacles. I don't know why it keeps happening, but it feels good."
Particularly after her last tournament. Kim took a three-shot lead into the final round of the ANA Inspiration two weeks ago, shot 75 and tied for fourth.
"After ANA, leading up to this tournament, I had a tough time sleeping because that was my first opportunity to win a major tournament on the LPGA Tour," Kim said. "I almost felt a little bit of guilt that I let that slip away.
"At the same time, I saw the possibility and the potential. I thought that I belonged there and that I can win a major championship in the future. So a lot of those lessons that I learned the last day at ANA Inspiration I was able to apply this week in the tournament. I think this helped me tremendously."
Hyo Joo Kim (69) and Chella Choi (72) tied for fourth at 7 under as South Korean players swept the first five places. German Sandra Gal (69) finished sixth at 6 under.
