Sunday's accident took place near the provincial capital of Balikesir at 04:40 local time (01:40 GMT) when the bus lost control and overturned.
"Eleven people died on the scene, three died at hospitals. Eighteen people were injured, they were sent to five hospitals", the government said in a statement.
The causes of the tragic crash remain unclear.
This was the second large accident involving a bus in Turkey over the past two days. On Saturday, nine people were killed and 30 others were injured when a bus crashed in the country's western province of Manisa.