Sgt. Paul Dawson says the boy and three adults were on a Ukrainian dance float in a parade to mark the town of Preeceville's centennial on Saturday.
The float was a dual-axel trailer being slowly pulled by a pickup truck.
Dawson says the boy, who was from Prince Albert, Sask., jumped off the front of the trailer on the passenger side and was run over by the wheels.
Two nurses who were on another float tried to help him, but he died in hospital.
Initial reports said the boy was seven years old, but RCMP say he had just celebrated a birthday on July 5.
It's not the first time a child has been killed by a parade float in Western Canada.
In July 1990, eight-year-old Mamie First Rider died after she fell under the wheels of a hay wagon she was riding on during the Calgary Stampede parade.