Around 4:10 a.m., an inmate notified a sheriff's deputy that his cellmate was unresponsive, sheriff's spokesman Jimmy Lee said.
Jail medical staff and deputies responded immediately and began life-saving measures on the inmate, a 46-year-old Pittsburg man.
Contra Costa County Fire Protection District crews arrived at the jail and continued to try to save the man, but he was pronounced dead at 4:27 a.m. Lee said the man had "several existing medical conditions."
His name is being withheld until relatives are notified.
The sheriff's office and Contra Costa County District Attorney's Office are investigating the man's death, following county protocol after in-custody deaths.