The vehicle, believed to be a lorry, was travelling from Aydin towards the Aegean city of Izmir when it flipped over and fell into the river several metres below, state-run Anadolu news agency said.
The nationality of the migrants was not made clear nor was it known where they were heading.
Turkish television pictures showed the stricken wreckage of the vehicle, which was reduced to burned-out metal by the impact of the crash.
Large numbers of emergency service personnel were sent to the scene.
Turkey is a key transit points for migrants from troubled countries in the Middle East, Asia and Africa seeking a new life in Europe via EU member Greece.
A million migrants crossed from Turkey into Greece in 2015, mostly by boats, in a crisis which forced a deal between Ankara and the EU to stem the flow of people.
Numbers have fallen since but people are still undertaking what is a highly perilous journey.
According to UN figures, more than 24,500 migrants have arrived in Greece by sea so far this year, with 118 people losing their lives via this route.