The coast guard continued searching for more possible victims and survivors.
The Merchant Marine Ministry said victims included four children.
The incident occurred early Friday, off the coast of Lefkada, an island in the Ionian Sea and the migrants were presumed to have been headed to nearby Italy, from the western Greek mainland.
Ministry officials said the migrants were aboard a plastic boat that was 7-8 meters (23-26 feet) in length. The survivors were being taken to hospitals for observation, four on the island and the others on the mainland.
“Coast guard boats and divers are continuing the search in the Palairos area, between Lefkada and the mainland,” Lefkada mayor Costas Aravanis told the AP.
“The boat sank after dawn in good weather conditions, with low prevailing winds, so it’s unclear how this happened. Some of the survivors managed to swim to the shore and call for help. They were not sure exactly how many people were on the boat so we still don’t know if there are others still out there.”
It was unclear where the voyage began.
Greece is the European Union’s busiest transit point for illegal immigration.