The Prisoner Ombudsman is investigating the death of a 19-year-old inmate who was on remand in the Young Offenders Centre at Hydebank.
Allyn Baxter died in hospital on Wednesday afternoon after being taken there at the weekend.
It is believed he had attempted to take his own life.
In a statement the Prison Service expressed "sincere condolences to the family and friends of the deceased".
An ombudsman's investigation is standard procedure when someone dies in prison.