The quake was also felt in the nearby Turkish provinces of Mugla, Izmir and Antalya.
ADVERTISEMENTA strong earthquake, with a preliminary magnitude of 5.9, struck off the southeastern Greek island of Rhodes on Wednesday, according to the Athens Geodynamic Institute.
There were no immediate reports of damage or injuries.
The quake was also felt in the nearby Turkish provinces of Mugla, Izmir and Antalya, with Turkey's AFAD emergency management agency saying it had a preliminary magnitude of 5.4.
It is common for different seismological institutes to record variations in magnitude in the initial hours and days after a quake.
There were no immediate reports of damage or injuries in Turkey either, although the earthquake caused panic in Mugla province, HaberTurk television said, with residents rushing out of buildings.
The undersea quake struck at 14:37 Greek time and had an epicenter nearly 400 kilometres southeast of Athens and 58 kilometres southeast of the Rhodes town of Lindos, the Athens Geodynamic Institute said.
Greece and Turkey lie in a highly seismically active region and experience hundreds of earthquakes each year.
The vast majority cause no injuries or damage.