The Idaho Department of Correction says Michael Allen Jauhola, 41, was moved to the medical wing of the prison in May.
Jauhola was on death row for killing a fellow inmate in 1998 while he was incarcerated for manslaughter and escape.
According to court records, Jauhola killed the other inmate with a baseball bat on the prison softball field.
IDOC says the Ada County Coroner will do an autopsy to verify the cause of death.
The nature of Jauhola's illness was not disclosed by the prison.
11 people, ten men and one woman, now remain on Idaho's death row.