The sheriff's office identified the boy as Jackson Roos.
Roos' mother thought he was riding his bike around their home when she went to look for him. She found him hanging from the zip line, said King County sheriff's spokeswoman Cindi West.
First responders attempted to revive Roos but he was pronounced dead at the scene, West said.
Investigators were examining how the boy got caught and on what, she said. They will look to see if his helmet's chin strap or the harness on the zip line played a role.
"We're not sure what he caught got on," West said. "At this point, it does appear to be an accident."
She said the zip line on the home's vast grounds in unincorporated King County is about 20 to 25 feet from the ground and about 100 feet long. It was professionally installed.
The zip line was removed from the home by detectives for the investigation.