Phuwadej Sukha, 16, jumped into the water at Kaeng Samchan on Saturday ignorant of a warning by officials not to do so due to the fast water flow.
The place is famous as a white-water rafting destination.
However, a recent storm increased the velocity of the water flow, and on Saturday officials warned tourists not to go into the water.
Phuwadej arrived at the spot with eight friends. They saw him swept away and immediately called for help.
As of press time, rescue workers were unable to find him.