Bomb-disposal officers carried out a controlled explosion on the device which was found during routine excavations in Glyfada, a suburb close to southern Athens.
Glyfada's mayor, Giorgos Papanikolaou, told Athina 984 radio station the bomb was a 95-centimetre explosive.
“It came from a German airplane during the 1941 bombardment of the area,” he added.
He said municipal workers stumbled upon the bomb while digging to build anti-flood defenses.
Residents were told to keep clear of the area during the controlled explosion but a general evacuation was not needed.
There were no reports of any injuries or damage after the controlled explosion.
In February this year, the Greek authorities were forced to evacuate 70,000 people in the northern city of Thessaloniki when another large WWII bomb was uncovered.