Police responded to a call at an apartment on King Street East near Victoria Avenue North around 11:30 p.m. to assist paramedics with a baby who was not breathing, said Staff Sgt. Emidio Evangelista.
The child later died in hospital.
Evangelista told CBC Hamilton that police do not know the cause of death, nor have they deemed it suspicious.
"Any time we get a call like this, we will investigate," said Evangelista, adding that the homicide unit is mandated to take over the investigation when it involves the death of a child under the age of five.
Hamilton police are waiting to hear the cause of death from the coroner's office.
The investigation is ongoing.