Police Lt Col Yutthasak Tonpho said the bus crashed into the side of the road and overturned today morning as it was rounding a curve while descending a hill in Kanchanaburi province. The driver was among the dead.
Thai media reported that the crash site is known to locals as "the curve of a hundred corpses" because of previous accidents there.
The passengers were on a holiday outing from a factory on the outskirts of Bangkok.
Thailand has the second highest traffic fatality rate in the world, according to a 2013 survey done for the World Health Organization.