The police said the bus, which departed Mae Sot district about 11pm, was taking about 50 Myanmar migrant workers back to Bangkok after spending the Songkran holiday in their home country.
At tambon Mae Tho, the bus ran off the road on a curve while going downhill and smashed into a roadside tree.
Chaen Noymee, 57, the Thai driver, was killed instantly. A man and a woman, both his relatives, sitting on the front seat were injured.
Also injured were 15 Myanmar workers. They were admitted to Taksin Maharaj Hospital in Tak provincial town.