The boy spotted the roughly 24 inch long cylinder with a red head casing in a wooded area of Berlin's Kopenick neighbourhood on Sunday, which his 62-year-old grandfather recognised as being a WWII-era bomb.
After being contacted by the grandfather, police bomb specialists arrived, confirmed it to be a WWII incendiary bomb, and removed it.
Berlin police plan to award the boy with a stuffed animal for his find, a spokesman said.