The 22-year-old from Quebec City defeated American Travis Stevens 1-0 in the men's 81-kilogram judo event to win bronze.
"It feels amazing. I've sacrificed so much and all of the fights today were very hard," Valois-Fortier said. "It was tough mentally but the whole team supported me and I managed to pull myself together. I wanted it really bad, it's what I work for every day."
It's Canada's first judo medal since Nicholas Gill won silver in 2000.
"I had to find inspiration from within," Valois-Fortier added. "My coach is very encouraging and I just kept listening and tried to get back into the contest."
In the final, Kim Jae-bum of South Korea won the gold, besting Ole Bischof of Germany in a final that he dominated from the start.
It was a reversal of the final of the Beijing Olympics, where Bischof won. This time, he said, Kim deserved the victory.
"He is much stronger and quicker now," Bischof said. "I think we have the correct champion."
Ivan Nifontov of Russia won the other bronze in the men's competition.
Valois-Fortier's medal came within 30 minutes of Canada winning a bronze in synchro diving.
Canada now has four total medals, all bronze.
Valois-Fortier was 4-1 in his early matches. He defeated Elnur Mammadli of Azerbaijan, Euan Burton of Britain, Srdjan Mrvaljevic of Montenegro, lost to Russia's Ivan Nifontov, then beat Emmanuel Lucenti of Argentina in the repechage to reach the bronze medal match.
"It was a really tough draw," Valois-Fortier said. "I started with one of the favourites — my first fight was with the world No. 3 (Mammadli) but I managed to pull it off."
Valois-Fortier's most recent win came May 20 at the Orenburg European Cup where he won a gold medal.
Valois-Fortier also won bronze at the Pan American Games in Mexico last year.
In the women's division, Urska Zolnir of Slovenia won the women's 63-kilogram class, defeating Xu Lili of China in a combative final where both fighters were almost constantly on the offensive.
The bronze medals were won by Yoshie Ueno of Japan and Gevrise Emane of France.
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