Paramedics were called to a house in Morden following reports of an unresponsive baby.
A 26-year-old woman's been arrested on suspicion of neglect after a five month old baby died on Wednesday 10th September.
A spokeswoman for the Metropolitan Police said; "We know the identity of the child and next of kin have been informed."
"A post mortem will be held in due course."
"The incident is being treated as suspicious.
"Officers from the Sexual Offences, Exploitation and Child Abuse Command are investigating."