The 20-year-old from Markham, Ont., crossed in 10.05 seconds, narrowly beating Ramon Gittens Barbados, who finished in 10.07.
Earlier in the night, Canada's Melissa Bishop won gold in the women's 800 metres. The 26-year-old from Eganville, Ont., ran to victory in one minute 59.62 seconds.
Toronto's Sarah Wells added a silver in the women's 400 hurdles.
De Grasse, who is coming off a remarkable performance that saw him win both the 100 and 200 at the NCAA championships, stepped up to the blocks to a huge roar from the packed 9,300-seat stadium at York University.
Also Wednesday, Canadian decathlete Damian Warner is on pace to break Michael Smith's Canadian record that has stood for 19 years.
The 25-year-old from London, Ont., takes a personal best score of 4460 into Thursday's Day 1.
The world championship bronze medallist opened with personal bests of 10.28 seconds in the 100 metres and 7.68 metres in the long jump.
He threw 14.36 in the shot put, the event that derailed his performance in May at the prestigious Hypo Meeting when he faulted on all three of his throws.
He then cleared 1.97 metres in the high jump — his best is 2.07 — and capped the night by winning the 400 metres in 47.66 seconds.
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