Andrea Dearden, Ada County Sheriff's Office public information officer, says the 155-millimeter military round was located in the back yard at a home on the 12700 block of Sonora Avenue.
Officials say the property owner found the round in the back yard of a recently-vacated rental house.
Dearden says the Boise City and Mountain Home Explosives Ordinance Disposal units are on scene and will dispose of the device. It's unclear how the device ended up where it did.
Officials had suggested voluntary evacuations for homeowners in the area, but most if all stayed in their houses.
Earlier this month, Elmore County officials say an Airman found an artillery round in the desert near the Snake River. It was later found to contain 15 pounds of explosives and was destroyed.