Choi, 17, scored a competition-high 90.25 in her third run at Livigno Snow Park to top Kim’s mark of 88.00, posted in her first run, with the American forced to settle for silver.
Japan’s Mitsuki Ono took bronze with a score of 85.00 in a final littered with crashes.
World Cup leader Choi recovered from a dramatic fall on her first outing, which resulted in a score of just 10.00.
She clipped the lip of the halfpipe, sliding down the wall, and lay motionless on the ice as medics went to her aid.
A stretcher was brought onto the snow but Choi regained her feet after a few minutes and drifted to the bottom of the halfpipe, cheered by a knot of South Korean fans.
Choi fell again early on her second run, leaving her Olympic dream at the Milan-Cortina Games hanging by a thread.
But she saved her best for last, landing a brilliant third run under intense pressure as the snow swirled down on the floodlit venue.
Kim, skiing last, gathered her thoughts at the top of the halfpipe, adjusting her equipment, but crashed early, confirming Choi as the champion.
“It’s the kind of story you only see in dreams, so I’m incredibly happy it happened today,” said Choi.
“During the final, mentally it was so tough but right now I am the happiest,” she added. “My knees are a bit bad, but I feel like I’m overcoming it all with happiness.”
Choi, the same age as Kim when the American won her first gold at the 2018 Pyeongchang Games, is the first athlete from South Korea to win an Olympic gold medal in snow sports. (AFP)
