At least seven people were injured in the accident, which took place on a motorway around 02:00 local time (00:00 GMT) near the village of Bosnek, south-west of the capital Sofia, local officials said.
The bus had a North Macedonian registration and the victims were returning to the capital Skopje from a weekend holiday trip to the Turkish city of Istanbul.
The cause of the crash is unknown, but an investigation has been launched into the incident. (ILKHA)