Asselin, riding Makavoy, posted a time of faultless time of 53.94 seconds for the 19-year-old's first win in the International Ring.
"It's surreal," said Asselin. "I've been dreaming about winning in the International Ring forever, let alone in the Masters tournament. The calibre of horses and riders is spectacular all week.
"I really came into this division as a true rookie. I haven't done that many tracks this height. I had no plan on winning anything. I have an incredible horse and he showed me he could do it."
Asselin was one of three riders out of 23 entries to advance to the jump-off in the driving rain.
Germany's Christian Ahlmann finished second while Belgium's Francois Mathy was third.