On Wednesday evening, Durham Regional Police were called to a home in Bowmanville, Ont., where a four-year-old boy was reported to be without vital signs.
Police say the child was then rushed to hospital, where he was pronounced dead.
Because the boy was under five years of age when he died, an investigation was triggered.
Police say that a post-mortem examination has determined that cause of his death was blunt-force trauma. His death is now considered a homicide.
No arrests have been made so far.
Police say the boy was being cared for at a family friend's home at the time of his death.
Anyone with information is asked to contact Det. Todd Dennis at 1-888-579-1520, ext. 5402, or Det. Dianne Jennings at 1-888-579-1520, ext. 5223.