The "very strong" quake hit at 0345 GMT with its epicentre recorded between the Peloponnese and the island of Milos, 145 kilometres (90 miles) south of Athens.
The Greek authorities said there were no reports of injuries or material damage.
An aftershock measuring 4.7 was recorded three minutes after the main quake, according to the observatory in the Greek capital.
Earthquakes are common in Greece
Thousands of people were left homeless in January by a quake on the Greek Ionian island of Cephalonia.
AFP