The tremor struck to the east of the Cycladean island of Andros, and about 125 kilometres (62 miles) east of the capital Athens, at 1614 GMT, according to the observatory.
No one was injured and according to the authorities so far there have been no reports of property damage.
Earthquakes occur regularly in Greece, a tremor on May 24 near the island of Lemnos caused injuries in neighbouring Turkey, while in January, thousands of people were left homeless by a quake on the Ionian island of Cephalonia, in the west of the country.