Jeremy Mainieri, 33, was found with a sheet around his neck at the Central Mississippi Correctional Facility.
Prison officials said it appears he hung himself!
Mainieri has been locked up since last July for the murder of Reverend Ed Everitt of Hammond while the priest was vacationing in Mississippi.
Authorities said Mainieri robbed and killed the priest, stole his car, and then tried to take his family to Disney World.
An autopsy is underway.