The news agency Ansa said the bomb was defused during a three-hour operation Sunday.
An alarm signalled the all-clear to re-open the city centre, as well as a nearby main road and a railway line connecting Italy with Austria and Germany.
The Neue Suedtiroler Tageszeitung identified the ordnance as an aerial bomb.
According to historian Ettore Frangipane, Bolzano, in the northern Alto-Adige region bordering Austria, suffered 13 major bombing raids that damaged 60% of the city and killed 200 people.
The region was part of a broad swathe of northern Italy that remained under Nazi-occupation long after Italy's 1943 Allied surrender.
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