The fatal blast occurred at about 3pm outside an office building of the Jarung tambon administration organisation in Yaring district as two rangers in the team were in the area.
The bomb, hidden underneath a marble bench adjacent to the office, went off when the rangers were sitting on it.
The explosion killed volunteer ranger Watchara Wailarb, 22, and seriously wounded another volunteer ranger Anusak Pinapong, 28. Anusak was admitted to Panare District Hospital and later pronounced dead.
The slain rangers regularly rested on the bench while on duty to provide safety for teachers at Ban Ma Pring School which is 70 metres away.
Forensic and explosive ordnance disposal (EOD) police officers said a 30-kilogramme bomb, packed in a cooking gas cylinder, was detonated by radio signal.