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
Couple shared tragedy, loss of Oso, but found love Woman shot, man arrested in Everett 12:42 p.m. Legal costs of Everett Public Schools internal investigation near $55K New trial date for father of high school shooter Lawsuit against Highland Christian School, teacher settled Arlington City Council fills vacant seat Everett Library’s music swap hits all the right notes Arlington woman dies in Skagit motorcycle crash
Our new comment system is not supported in IE 7. Please upgrade your browser here.