The man was housed in the jail's medical unit, said Shari Ireton, spokeswoman for the Snohomish County Sheriff's Office, which runs the jail.
Corrections staff found the man "not breathing and unresponsive," she said.
Paramedics were summoned.
A death investigation by sheriff's detectives was under way Thursday.
The body was transferred to the Snohomish County Medical Examiner's Office, which will confirm the man's identity and determine how he died.
At least seven people have died at the county jail since 2010.
More Local News Headlines
Contract for Snohomish Co. Jail to take Seattle inmates approved 4:26 p.m. Voters overwhelmingly chose Sultan's newest councilman, 21 Woman struck by car on Highway 99 in Edmonds Hearing set on car allowances for Snohomish County officials Burn ban lifted in Snohomish County — except Darrington Mukilteo School District ponders change as enrollment continues to surge Monroe High's Cameron Lee wants to pursue theater, music interests Community Calendar
Our new comment system is not supported in IE 7. Please upgrade your browser here.