A tour guide drowned when a flash flood swept into Nam Talu cave in Khao Sok National Park on Tuesday evening.
The 22 foreign tourists and a second guide escaped without serious injury.
The cave is closed to the public from June to November each year because of the danger of flooding.
It was not known why the guides ignored this and took the tourists inside on Tuesday.
A flash flood poured into the cave about 7.30pm as the group of 22 tourists and two guides were walking out of it.
One of the guides was washed off his feet and carried back into the cave. The others in the party were able to resist the current and were led to safety by rangers. Two Dutch women received minor leg injuries and were treated at a local hospital.
Rangers started a search for the missing guide. His dead body was found inside the cave about 3.20am on Wednesday.
