The 36-year-old man was found with head injuries at the jail in south London on Tuesday night.
He died in hospital on Wednesday evening and detectives await a postmortem to establish the cause.
Two other inmates, both aged 28, were arrested at the scene on suspicion of attempted murder and remanded in custody.
“We are working to establish the circumstances surrounding this incident with the support of [private security firm] SERCO and local officers,” said Detective Chief Inspector Richard Leonard.
“At this stage I am open minded concerning motive, but expect this to become clear as the investigation progresses.”
HMP Belmarsh is notorious for holding some of the most dangerous prisoners in the UK, as well as Wikileaks founder Julian Assange.
And after recent terror attacks, fears were raised that it could be acting as a “breeding ground” for jihadists. However, there has been no indication Tuesday’s attack was a jihadist or violent extremist incident.
Officers were called to the prison following reports of an assault, according to a statement from the Metropolitan Police.
“An investigation has been launched following the suspicious death of a prisoner inmate at HMP Belmarsh,” the force said.
“Police were called at 20:00hrs on Tuesday, 18 February to reports of an assault within the prison facility.
“Officers and the London Ambulance Service attended the location where a 36-year-old man was found with head injuries.
“The injured man was taken to hospital in a critical condition where he later died on the evening of Wednesday, 19 February. His next of kin are aware.
“A post-mortem examination will take place in due course.”