The quake hit at 00:30 local time (2130 GMT) offshore 45 kilometres (28 miles) northeast of the Greek capital, said the Institute of Geodynamics at the National Observatory of Athens.
The epicentre was four kilometres off the seaside resort of Nea Styra in the southwest of Euboea, Greece's second-biggest island, the institute said.
There were no immediate reports of casualties or damage.
The mayor of the nearby city of Marathon, Stergios Tsirkas, described the quake as "very intense", in comments on ERT television.
In May, a strong earthquake of magnitude 6.1 struck off the Greek island of Crete and was felt as far as Egypt as well as in the Greek capital.
In January and February, the island of Santorini in the Aegean Sea, a major Greek tourist destination, suffered exceptional seismic activity.
Thousands of tremors there caused several thousand residents to flee, but they have since returned home.
Situated on several faults in the southeastern Mediterranean, Greece is regularly hit by earthquakes.
The last deadly quake there struck in October 2020 on the island of Samos, in the Aegean Sea.
With a magnitude of seven, it killed two people in Samos and over 100 in the Turkish port city of Izmir.
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