Police said an 18-year-old runner suffered an unspecified medical problem while taking part in the race, but would not release other details.
Emergency medical personnel told CBC they responded to a call at about 11:08 a.m. near Lakeshore Boulevard and Dowling Avenue, where they found an 18-year-old woman without any vital signs.
People on the scene were performing CPR and she was rushed to hospital, they said.
The Cape Breton Post identified the runner as Emma van Nostrand, and said her parents, Steve and Katherine van Nostrand of Coxheath, N.S., were also in Toronto competing.
Steve van Nostrand had recently returned from the Boston Marathon, where he had completed the race for the seventh time in a row.
Van Nostrand was a Grade 12 honour student at Riverview High School, where principal Joe Chisolm said a crisis intervention team would help students deal with her death, the Post reported.
Quebec man, woman fastest to finish
The marathon steered thousands of runners through the city’s downtown core, though not everyone was running the full 42-kilometre course.
More than 1,600 people finished the full marathon on Sunday, with runners from Quebec winning both the men’s and women’s races.
Mylene Sansoucy of Montreal finished the marathon in two hours and 58 minutes.
Sansoucy told CBC News that she was "very happy" with her result.
"I won the marathon and I’ve done my best time," she said Sunday.
Terry Gehl of St Charles-Sur-Richelieu, Que., was the first man to cross the finish line. He finished in about two hours and 37 minutes.
Gehl, who was coincidentally wearing bib No. 1, said he was coming back from an injury that prevented him from running the marathon last year.
Results posted online showed that another 4,400 people ran the half-marathon and more than 1,400 did a five-kilometre run.
The people participating in the full marathon started their run at Mel Lastman Square in North York and followed a course that took them along Yonge Street, through part of mid-town Toronto, along Rosedale Valley Road, down Bayview Avenue, through the downtown and along Lake Shore Boulevard looping back to Exhibition Place.