A 25-year-old man has been murdered at Coldingley prison near Woking, Surrey.
The Prison Service named him as Madala Washington.
"As with all deaths in custody there will be an investigation by the independent Prisons and Probation Ombudsman," said a spokesman.
The category C training prison holds 500 inmates, who cannot be trusted in open conditions but are considered unlikely to make a determined 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."
The Ministry of Justice says Coldingley, based in the village of Bisley, "is focused on the resettlement of prisoners ... offering opportunities to prisoners willing to work hard and accept responsibility for achieving those goals."