At 4.15am police were informed a baby had been bitten by a dog inside an address in Falkland Road in Sunderland. The three week old baby boy was taken to hospital where he later died.
The dog, a small terrier, has been removed from the address and is likely to be destroyed.
Enquiries are on-going to establish the circumstances surrounding the incident.
Superintendent Gillian Mitchell said: “This is obviously a very tragic incident and our thoughts are with the family of the baby at such a difficult time.
“We are carrying out enquiries to establish the circumstances of the incident and will have extra officers in the area to allow this to be done.
“Officers will also be on patrol to offer reassurance to the local community who may be concerned.”
A 30-year-old man has been arrested in connection with the incident and is helping police with their enquiries.