Reports say some 46 others were injured in the accident.
The bus was travelling overnight from the capital, Ankara, to the Uludag resort near the city of Bursa.
It crashed into trees in the region of Eskisehir in what were said to be good road conditions, reports say.
Eskisehir governor Ozdemir Cakacak, quoted by the Dogan news agency, said it was not clear what caused the accident as the road was not wet or frozen.
Those injured were currently being treated at three different hospitals, he added.
It is not known if there are any children among the victims.
The two drivers, who were slightly injured, have been arrested, and an investigation has been launched, according to Dogan.
Turkey has a poor track record in road safety, with more than a million accidents in 2016 and 7 300 deaths, according to official statistics, the AFP news agency reports.