The 2315 GMT quake had its epicentre 230 kilometres (143 miles) southwest of Athens in the vicinity of the Cythera islands, south of the Peloponnese peninsula, the institute said.
Two stronger quakes measuring 6.5 and 6.4 Richter struck on February 14 in the same area which is highly seismically active.
Seismologists say that Greece, which accounts for half the seismic shocks recorded on the continent, experiences small tremors almost every day.
Seismic activity has been heightened this week, with a 4.4-Richter tremor recorded near Mount Parnitha northwest of Athens on Sunday and another registering 4.0 in the same area on Tuesday.
The Mount Parnitha area was the epicentre of a 5.9-Richter quake that killed 143 people in Athens in 1999.
Güncelleme Tarihi: 20 Şubat 2008, 15:22