A minivan belonging to a human trafficker and transporting 16 refugees mainly from Iraq and Syria crashed into a wall and overturned on a highway near the town of Nea Karvali on Friday.
Police said the vehicle’s driver, the trafficker, was also killed in the crash.
"Possibly the driver lost control of the vehicle, the investigation is ongoing," an unnamed police spokesman said, Presstv Reported.
A hospital official said three people were also injured in the crash.
The nationality of the driver was not immediately clear. However, police added that the car had a Polish number plate and it was not being pursued.
Greece has been the main gateway for refugees into wealthier European states.
Europe has been facing an unprecedented influx of refugees, most of whom are fleeing conflict zones in North Africa and the Middle East, particularly Syria. Many blame major European powers for the exodus, saying their policies have led to a surge in terrorism and conflicts in the Middle East, displacing the locals.