A 38-year-old remand prisoner at Maghaberry jail died on Wednesday morning.
The man’s next of kin have been informed by the prison service but no other details have been released.
The PSNI, coroner and Prisoner Ombudsman have begun investigations into the death.
Head of the Prison Service Ronnie Armour said: “I would like to extend my sympathy and that of the Prison Service to the family of the man who has died.
“My thoughts are with them at this difficult time.”