Compatriot Funa Nakayama, 16, finished third to bag the bronze medal. The podium was dominated by teenagers, as Brazil’s Rayssa Leal, also 13, took silver.
Nishiya finished first in the eight-person final with 15.26 points. Leal scored 14.64, while Nakayama earned 14.49.
Nishiya started slowly in the final, but recovered to score high marks in her last three tricks.
“At first, I thought there’s no chance to win, but people around me encouraged me,” Nishiya said.
“So I shifted my focus, and determined to never give up until the end.”