The boy came across the grenade in the communal bin of his block of flats in Knittelfeld in southern Austria on Sunday evening. It was neutralised by police explosives experts.
"Even rusty war relics and weapons can still be dangerous," a police statement said. "Don't touch and keep a safe distance. Call the police emergency number immediately."
More than 70 years after the war ended, weapons and unexploded ordnance still regularly turn up in Austria and Germany, particularly during construction work.
In 2010, three German sappers were killed defusing a wartime bomb in the central city of Goettingen.
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