Pol Capt Kiarttisak Boonplong, deputy inspector of Mae Thor police station, said 28 of the injured workers were in severe condition in hospital.
He said the accident happened at 10.51pm on the Mae Sot-Tak road at the kilometre marker No 68+200 in Tambon Mae Thor in Muang district.
The double-decker bus was transporting 50 Myanmar migrants from the the border checkpoint in Tak's Mae Sot district to Chachoengsao's Plaeng Yao district.
Pisit Thanathakpasit, 40, the bus driver told police he could not slow down at the curve. He said he attempted to drive the bus against the roadside concrete barrier to reduce speed but it eventually plunged off the road and crashed about five metres down the hillside.
Police suspect the driver was speeding and his brakes failed.