Pol Captain Chaiwat Toonchaiyaphum, deputy inspector of Chaturas police station, said the accident happened at 4:30am on the Chaiyaphum-Si Khiew road.
One of the passengers was severely injured and was rushed to the district hospital.
The bus driver, Phochai Sinthucharoen, said a pickup changed lane and cut in front of the bus, prompting him to swerve to the left and drive off the road. He said it was raining at the time and the road was slippery.