The bus, which was heading to the eastern Agri province from Istanbul, continued to slide on its side until it was stopped when it slammed into a water fountain on the side of the road, witnesses told The Anadolu Agency.
Victims were transferred to area hospitals and at least four people remain in critical condition.
The driver suffered no injuries and was taken into custody by police.
An initial investigation said the front tires of the bus were not the right type for the winter season.
Winter tires are a requirement for all heavy vehicles in Turkey, including trucks, tow trucks, tankers and buses.