The body of a foreign man was found floating off Jomtien beach, near Pattaya, on Tuesday night.
He was earlier seen walking on an off-shore floating pontoon.
The man, estimated to be 25 years old, was found face down in the water opposite Soi 7 on Jomtien beach road about 10pm on Tuesday.
Efforts to resuscitate him were unsuccessful, Khaosod Online reported on Wednesday.
Police found no documents on his body to identify the dead man. He was believed to be a European national.
Witnesses told police they earlier saw the man walking on a floating pontoon, opposite the entrance of Soi 8, Jomtien beach road, near where his body was found.
Police sent his body to a hospital for a post mortem examination. They were trying to track down his identity and would then notify the appropriate embassy.