The Wanaka teenager won the slopestyle at the US Open Championships in Colorado on Saturday.
It capped an impressive year in which she also won gold at the X Games in Aspen and a world championship title.
She had entered Saturday's finals as the top qualifier and jumped straight to the top of the leaderboard with a score of 82.55 on her first run.
It proved to be the winning score, conditions becoming tougher on the second runs and second-placed Julia Marino taking a fall on her third jump.
"I can't even explain how I'm feeling right now," Sadowski-Synnott said.
"It means so much. It was one of the first contests I watched when I started snowboarding and I'm just so happy to be here.
"It's been an amazing season and I wanted to finish it on a high and I'm just so happy to do it here at the Open."
