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.
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