Sources at the Co Antrim prison say he committed suicide.
The man has not yet been named.
But is understood he had been convicted a number of years ago of murder.
In a statement, the Prison Service: “We can confirm the death of a prisoner at Maghaberry prison.
“His next of kin have been informed and the Prison Service has expressed sympathy to the family.
“The PSNI, coroner and prisoner ombudsman will conduct a full investigation into the death.”
It is the second death at the high security prison in less than a fortnight.
Gerard Mulligan took his own life on November 5.
He was on remand charged with murdering his father, Gerald Mulligan, in September with a breeze block at his Lisburn home.