He died at University Hospital of Wales in Cardiff on Thursday afternoon after being found in a critical condition at an address in Broadlands, Bridgend, on Wednesday at around 8.30pm. A 31-year-old woman has been arrested on suspicion of murder, South Wales Police said.
The suspect remains in police custody.
A force spokesman said: “South Wales Police’s major crime investigations team have launched a murder investigation following the death of a two-year-old boy at the University Hospital of Wales in Cardiff this afternoon.
“The child was admitted after being found in a critical condition at an address in Broadlands, Bridgend at approximately 8.30pm on Wednesday night.
“A 31-year-old woman has been arrested on suspicion of murder and remains in police custody.”
A Welsh Ambulance Service spokesman said: “We were called at 7.47pm on Wednesday, August 11, to reports of a medical emergency in the Broadlands area of Bridgend.
“We responded with multiple resources including an emergency ambulance, a rapid response vehicle, our hazardous area response team, and emergency medical retrieval and transfer service.”
Detective Chief Inspector Matt Davies stressed that officers were not looking for anyone else in connection with the incident.