The 48-year-old inmate was found unresponsive early Wednesday at High Desert State Prison in Susanville.
Prison Lt. Nick Albonico was not authorized to say why the death was considered suspicious. The 41-year-old cellmate was being kept in isolation during the investigation.
Both men were from Los Angeles County, and neither is being named by authorities.
The inmate was serving a 22-year sentence for assault with a deadly weapon, while is cellmate is serving a 28-year sentence for rape and other charges.
The prison is 185 miles northeast of Sacramento.
It was the second such death this week. An inmate died Monday at Calipatria State Prison in Imperial County.