The Regina native scored 96 points on a monster third run for his 17th X Games medal, one-off American Shaun White’s record.
McMorris, a two-time Olympic bronze medallist in slopestyle, was coming off a silver-medal performance in big air Friday night.
The 25-year-old scored 89.66 points on his first run down the slopestyle course on Saturday and 21.33 on his second. The final score is the best of three runs.
Finland’s Rene Rinnekangas won silver with 94 points and Mons Roisland of Norway took bronze with 91.33.
Sebastien Toutant of l’Assomption, Que., appeared to have stamped his spot on the podium with a 90.66-point third run that put him in position for bronze. He was bumped with McMorris’s final score.
Toutant, the big air gold medallist from the Pyeongchang Olympics in 2018, also placed fourth in Friday’s big air competition. Darcy Sharpe of Comox, B.C. finished seventh in big air and sixth in Saturday’s slopestyle event.
McMorris has eight X Games gold medals to go with six silver and three bronze.
In the men’s ski big air Saturday evening, Quebec City’s Alex Beaulieu-Marchand finished with a combined score of 87 to capture silver behind Norway’s Birk Ruud (89) and ahead of Britain’s James Woods (82). Evan McEachran of Oakville, Ont., came in fourth with a score of 81.
The 24-year-old Beaulieu-Marchand picked up a bronze for Canada at Pyeongchang’s slopestyle competition.
Earlier in the day, Spencer O’Brien finished just off the podium in the women’s snowboard slopestyle.
O’Brien, from Courtenay, B.C., earned 81 points on her second of three runs down the course.
New Zealand’s Zoi Sadowski-Synnott won gold with 91 points. American Hailey Langland was second with 90.66 and Enni Rukajarvi of Finland took bronze with 84.66.
Stoneham, Que., rider Laurie Blouin, the 2018 Olympic slopestyle gold medallist, was seventh in the eight-woman final.