The Prison Service said the man was a remand prisoner and died on Saturday evening.
The prison service have not released any details about how he died but said the man’s next of kin have been informed.
The PSNI, coroner and Prisoner Ombudsman have launched investigations into his death.
Acting Prison Service director general Phil Wragg said: “I would like to extend my sympathy and that of the Northern Ireland Prison Service to the family of the prisoner who has died in Maghaberry.
“My thoughts are with them at this difficult time.”
It is the latest in a number of deaths at the Co Antrim jail.