The accident occurred around midnight Thursday-Friday in Lom Sak district, Phetchabun province while the bus was en route from Khon Kaen to the northern province of Chiang Rai.
The bus crashed through the railings of the Por Khun Pa Muang bridge and hurtled into the ravine below.
"We suspect the bus driver fell asleep," Major General Sukit Samana, police commander of Phetchabun province, told AFP.
The bus had a total of 30 passengers, a driver and two staff.
A total of 29 people died at the scene, 20 men and 9 women. The other four were in critical condition at a hospital.
"The eyewitness who informed the police said the bus went very fast before it plunged into the ravine," he said.
The bus was completely destroyed in the accident. Almost 200 rescue personnel were involved in the operation to recover the bodies from the ravine, about 50 metres below the bridge.
The accident occurred as millions of Thai motorists are expected to travel during the New Year holidays.
Deadly road accidents are common in Thailand. A total of 365 people were killed and 3,329 others injured in 3,176 traffic accidents across the country during the 2013 New Year period.
At least 20 people were killed in October when a tour bus carrying elderly Buddhist devotees plunged into a ravine in Lampang province in northeast Thailand.
A recent report by the World Health Organisation said the country saw some 38.1 road deaths per 100,000 of population, compared to an average of 18.5 in Southeast Asia as a whole.