Halton police say two suspects confronted the man as he was running a leg of the relay along Lakeshore Road West in Oakville, early Saturday morning.
The suspects demanded the man's wallet and showed him a hand gun — which was later determined to be a pellet gun — before pushing him to the ground, police said.
According to a news release others runners saw what was happening and attempted to help.
"One of the accused turned and threatened the other runners with the gun before he fired several shots at both runners ... one female victim was grazed across her nose by a pellet and luckily all the other shots missed. No medical treatment was needed," police said.
The suspects ran away but were caught a short time later by police.
Two 18-year-old men from Oakville face robbery charges.
All of the victims were from the United States and were visiting Ontario to take part in the Ragnor Relay Race between Cobourg and Niagara Falls.
Police say the runners "all continued with the race."