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
Study says Paine Field should be used for business, not museum Judge to decide future of sex crime case against ex-police sergeant Tamper-proof cigarette bins may cut down on litter in downtown Everett Suspect sought in fatal Lynnwood shooting 'The voice of reason': Mill Creek Police Chief Bob Crannell to retire Camano Island Fire and Rescue seeks volunteers; meeting Oct. 13 Arlington man identified as rider in fatal motorcycle crash Front Porch: Help plant rain gardens Saturday in Edmonds
Our new comment system is not supported in IE 7. Please upgrade your browser here.