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
After audit, Lynnwood is fixing its financial problems Hit and run in south Everett leaves woman in critical condition Students impressed by career opportunities at Manufacturing Day 3-D technology finds some hidden cancers but can cost more Public Disclosure Commission calls Eyman's violations serious Arlington man dies in crash 7:06 p.m. Corey McCrea, 33: Snohomish skateboarder had found his gift Complaints say that commerce officials helped PUD score a grant
Our new comment system is not supported in IE 7. Please upgrade your browser here.