Austin Reamer found the grenade Sunday night in the basement ceiling of his family's rental home in Milwaukie, The (Portland) Oregonian reported.
Austin immediately told his father, Christopher Reamer, who alerted authorities and put the grenade on his front lawn for police.
Police treated the emergency as if they were dealing with a live grenade but did not disclose if the device turned out to be active.
When officers arrived, they closed portions of the Reamers' neighborhood and asked people in the area to remain indoors while they investigated.
The grenade was identified as a "modified military training grenade," a news release from the Portland Police Bureau's Explosive Disposal Unit said. Officers later disposed of it.
Police did not say who might have put the grenade in the house.