There were no indications on the asphalt that Thaweesak Chidkua, 36, had applied the brakes before striking a road sign and careening off south-bound Phetkasem Road near the Ban Luek-Haad Phan Krai intersection in Chumphon at about 4.45am.
Thaweesak, ferrying 11 male and 12 female tourists from the Philippines, suffered a broken right leg and was trapped in his seat until rescuers could extract him and take him to hospital.
A woman passenger went along, suffering from a head injury.
The other tourists were moved onto another bus to continue their journey to Phuket.
Police planned to charge the driver with reckless driving causing injury.