The tragedy took place at about 2:00 pm (0700 GMT) at the Sairung waterfall in Trang province, some 700 kilometres (434 miles) south of Bangkok.
"At least 23 people were confirmed dead and we have learned that more bodies are coming," a local hospital official said, adding that she did not know how many people were missing.
Most of the victims were women and children, she said.
A senior provincial official said 23 people were killed and at least 24 were missing several hours after the accident, while police major general Kachorn Siriwan, in Trang province, said earlier at least 14 people were still missing.
"People were enjoying the waterfall and suddenly a flash flood swept over," the police officer said.
The Nation English language daily said on its website that tourists at the waterfall were caught off-guard by the flood and many failed to "get out of the waterways in time."
Several injured tourists were stranded on rocks and trees waiting for rescue workers, the Nation said.
Local reports also said the area had been hit by heavy rains since Thursday.
Following the incident, hundreds of rescue workers, including military personnel, local police and volunteers, were mobilised to hunt for the missing and to help survivors, the senior provincial official said.
"We are continuing our search and rescue operations," he said.
Around 100 holidaymakers were enjoying the waterfall at the time of the tragedy, according to the local hospital official.
Thailand is celebrating the five-day Buddhist New Year holiday, which started on Friday.
Last Mod: 20 Eylül 2018, 18:16