The accident occurred around 3:00 a.m. local time when the bus, which was on a study tour, lost control and veered off the road, the province’s public relations office said.
According to a local road safety center, 16 people died at the scene, while two others succumbed to their injuries later. The injured passengers were transported to a nearby hospital for treatment.
"It was a downhill road and the brakes failed, and the driver lost control of the vehicle before it overturned," said Colonel Sophon Phramanee. He added that those killed were adults participating in the study trip.
Images shared on social media showed emergency responders assisting victims near the overturned bus, which lay on its side with its undercarriage exposed.
Thai Prime Minister Paetongtarn Shinawatra extended her condolences to the victims' families and stated that an investigation was underway, focusing on vehicle safety.
"If it is found that there is a violation of the use of vehicles that do not meet the standards or are involved in reckless use of vehicles, legal action will be taken," she wrote on X.
"Inspection of vehicles must be safe and pass the specified standards before they are put into use to prevent accidents and reduce losses like this again," she added.
Road accidents are frequent in Thailand due to weak enforcement of vehicle safety regulations and poorly maintained roads.