Seattle police spokeswoman Renee Witt said the man climbed a fence and was found inside a secure area at about 9:50 p.m.
Staff at the jail said the man was trying to open a locked door, but never made it inside the building.
Witt said the man told police he knew he was at the jail, but did not say why he was trying to get inside.
He did eventually get in the building, but maybe not the way he wanted. Officers arrested the man and booked him into the jail for investigation of criminal trespass.