A prisoner has been arrested over the murder of an inmate at Coldingley prison near Woking, Surrey.
Officers were called to the jail shortly after 1pm following reports of a "serious assault".
The 25-year-old victim was named by the Prison Service as Madala Washington.
Police said a 23-year-old man had been arrested on suspicion of murder and remains in police custody while an investigation is carried out.
The Prisons and Probation Ombudsman will also investigate the death.
The category C prison holds 500 inmates, who cannot be trusted in open conditions but are considered unlikely to escape.
Steven Gillian, 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.
"It is tragic that someone has lost their life."
There were 357 deaths last year in UK prisons, including eight murders, the highest figure since records began in 1978.