The driver of the Bangkok-Si Sa Ket bus, Somyos Thammawat, 41, was severely injured in the crash.
Police said the accident happened at 3.35am on highway No 24 near the Ban Nong Waeng intersection in Tambon Nong Tako, Sung None district.
The injured passengers told police that the bus swayed before it fell into the ditch and overturned.
Police believe the drive dozed off behind the wheel.
The injured were taken to the district hospital.