Greek authorities said a plastic boat carrying refugees overturned near the coast of the eastern Aegean island of Kos on Tuesday, killing at least eight people and leaving four others missing.
A coastguard official added that emergency crews were still trying to find the bodies of the people who died.
Seven refugees were rescued. Crews are still searching for three to five people listed as missing.
Europe is facing a huge refugee crisis, during which more than 600,000 people have made their way through Greece so far this year, running away from war and poverty in their countries.
Greece, tackling its own economic crisis, is used as a route that particularly Syrian refugees take to cross into other European countries.
The vast majority of refugees do not stay in the country, as Greece has said it does not have the funds to care for all the arrivals.
On their way to a new life, more than 500 people have been killed or have gone missing since the beginning of the year.
Greece's Prime Minister Alexis Tsipras is visiting Turkey on Tuesday and Wednesday to discuss the worst refugee crisis since World War II.
Tsipras had said before that financial aid could be offered to Turkey to register and process the refugees on Turkish territory instead of allowing them to stay in Greece.