Shortly after midnight on Sunday, an Ontario Provincial Police officer with the Nottawasaga detachment tried to stop a motorcycle in the area of 8th Avenue, according to the SIU.
A short time later, the motorcycle hit a construction barrier, the SIU said.
The motorcyclist, a 34-year-old man, was taken to Stevenson Memorial Hospital in Alliston and was pronounced dead.
The SIU is an arm's length agency that investigates death, serious injury or allegations of sexual assault in which police are involved.
Anyone with information is asked to contact the lead investigator at 1-800-787-8529 ext. 1935.