The attack happened at 11:15 p.m. ET, when two men were walking a dog along Burton Street in central Hamilton, they say.
"The dog began to attack one of the men and he collapsed," a police news release reads. "A passerby intervened and using a baseball bat, attempted to fend off the dog."
The man was pronounced dead at the scene. His official cause of death has not yet been determined, but a post-mortem is scheduled for Thursday.
Animal control has possession of the dog, police say. Residents have described it as a pit bull.
Fire crews were called to the scene as well, but were called off before they had any contact with the man.
"We had the information that it was a dog bite," and he had no vital signs, Mostacci said.
Burton Street is closed between Victoria and Emerald as police investigate.