An inmate has been killed at Coldingley Prison in Woking, police said.
Madala Washington, 25, died after being attacked at the Bisley prison around lunchtime on Friday.
Surrey Police said he had been the victim of a "serious assault" at the prison, which houses around 500 inmates.
A spokesman said: "Officers attended the address shortly after 1pm today following reports of a serious assault on an inmate at the prison.
"Detectives are carrying out urgent inquiries to establish the circumstances surrounding the incident."
The Prison Service added: "HMP Coldingley prisoner Madala Washington died in prison on Friday April 1.
"As with all deaths in custody there will be an investigation by the independent Prisons and Probation Ombudsman."
Coldingley is a category C training prison, meaning inmates are considered unlikely to make a determined escape attempt but cannot be trusted in open conditions.
It has capacity for around 500 prisoners in five wings, where inmates are mainly housed in single cells, and is focused on the "resettlement of prisoners".
The death comes against a backdrop of concerns about rising cases of violence in UK prisons.
There were 257 deaths in custody last year including eight homicides, more than in any other year since records started in 1978.
In 2015 Coldingley's independent monitoring board wrote to prisons minister Andrew Selous over its concerns about the jail.
Its report blamed an increase in home-made weapons at the prison and illegal drugs on cuts to the number of prison officers and staff.
Steve Gillan, general secretary of the Prison Officers Association, said: "We do not comment on ongoing police investigation, but generally there has been an increase in violence, and indeed homicides, in our prisons.
"We believe that is due to the lack of prison officers - some 7,000 have been made redundant since 2010.
"Our prisons are a more violent place than they have been. It is tragic that someone has lost their life."