The victims were determined to be four Swiss citizens, a British man and a 10-year-old German boy, said police Lt. Col. Pichan Kanayasiri.
The group had been trekking through the Khao Sok national park in Surathani province late Saturday afternoon when a heavy rainfall caused flash floods that sent water surging through the cave, which was near a waterfall, Pichan said.
"The tourists were inside the cave and didn't know what was happening outside," Pichan said. "They were trapped inside the cave."
A British woman, who survived by clinging to the ceiling of the cave, was found alive after a long search, Pichan said.
The province's governor, Winai Phopradit, said he had ordered the national park to close during the current rainy season.
"We have signs both in English and Thai warning tourists not to go into the cave during heavy rains," he said.
A Belgian tourist was killed by a flash flood in the same cave several years ago.