The earthquake had its epicentre near the town of Pyrgos and a depth of just ten kilometres (6.2 miles), the institute said.
According to local authorities it was felt in the outlying area as far as the Ionian islands.
Greece is one of Europe's most earthquake-prone countries. In November, a 6.0-magnitude earthquake killed two people on the Ionian island of Lefkada and damaged roads and buildings.