The inmate's name has not been released.
According to a press release, the 34-year-old man was found unresponsive in his cell during a routine inmate check just before 4 a.m.
Officials said that corrections officers performed CPR but were unable to revive the man.
The inmate's body was taken to the state medical examiner's office to determine a cause of death.
The Maine State Police will conduct an independent investigation.