The quake's epicentre was located in the vicinity of the town of Vartholomio, in the north-western Peloponnese (western Greece), and was felt throughout the immediate area, an institute source told AFP.
No damages or injuries were immediately reported.
Vartholomio had been hit by earthquakes of similar strength in 2002.
Greece is Europe's most quake-prone country, accounting for half the continent's seismic activity.