The truck was travelling on a highway in the Izmir region close to Izmir airport on Sunday when it flipped and fell into the channel several metres below, Anadolu news agency said.
Turkish television showed the wreckage of the vehicle with corpses strewn nearby; the truck had been reduced to burned-out metal by the impact of the crash.
Thirteen people were injured and taken by ambulance to nearby hospitals, according to local media.
DHA news agency said the driver of the vehicle, a Turkish man aged 35, survived and told police from his hospital bed that he had swerved to avoid a white oncoming vehicle.
The dead included two babies and two children as well as a pregnant woman, DHA reported.
The truck’s passengers were foreigners who had reached a deal with traffickers to be transported by boat to the Greek island of Samos, south of Izmir.
Turkey became one of the main launch points for more than a million refugees and asylum seekers taking the sea route to EU countries in 2015, many fleeing conflict and poverty in the Middle East and Africa.
The influx of people was drastically curtailed by a 2016 accord between Ankara and the European Union after hundreds died crossing to Greek islands a few miles off Turkey.
According to UN figures, more than 24,500 migrants and refugees have arrived in Greece by sea so far this year, with 118 people losing their lives via this route.
Last week, eight people were found drowned off the Karaburun district, also in Izmir province, after their boat capsized.
Twenty-six others are still officially listed as missing after that accident, according to Anadolu.