The teenager has been touted as a star of the Games in South Korea because of her prodigious talent, ever-ready smile and Korean heritage.
Kim -- the heavy pre-tournament favourite -- claimed a comfortable victory with a winning score of 98.25, well ahead of China's Liu Jiayu, who took silver, with Arielle Gold of the United States earning bronze.
The United States are threatening to dominate the snowboarding in South Korea.
Kim's victory made it a hat-trick for Team USA after Jamie Anderson won a chaotic women's slopestyle final and another talented teenager, the 17-year-old Red Gerard, topped the podium in men's slopestyle.
Kim has become a darling of the home fans in Pyeongchang, who have adopted her as one of their own because her parents are Korean.
The four-time X Games champion would have challenged for gold in Sochi four years ago -- if she hadn't been too young to compete.
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