The Prison Service has confirmed the death of a prisoner from Magilligan in Co Londonderry.
The 27-year-old died on Monday morning in hospital. His next of kin have been informed.
An investigation has been launched, as is standard procedure.
Director General Ronnie Armour said: "On behalf of the Northern Ireland Prison Service, I would like to extend my sympathy to the family of the prisoner.
“My thoughts are with them at this difficult time."