The body of a foreign man was found on Mae Ram Phueng beach in Rayong on Sunday.
Pol Maj Sakol Khamyingyong of Phe police station said the body of a foreigner had been reported on the beach in Muang district.
Police found no signs of a physical attack on the body, and believed that he died about three hours before local residents alerted authorities at 7.30am.
Police speculated that the man had drowned while swimming in the sea, with the tide washing the body ashore.
Pol Lt Col Ek Mahasawad, a tourist police officer in Rayong, said police were contacting all hotels and resorts in the area in an attempt to identify him.