Late Silurian to Early Devonian ostracods are reported for the first time from the Kuan Tung Formation (Sibumasu terrane) in southern Thailand. The studied limestone samples yielded abundant ostracods belonging to five suborders, six superfamilies, 17 genera, and 34 species, including the newly described Ampuloides thungsamedensis nov. sp. Other species are members of Ampuloides, Microcheilinella, Bairdiocypris, Baschkirina?, Rectella?, Pseudorayella, Acratia, Bairdia, Fabalicypris, Cytherellina, Healdia, Kummerowia?, Aparchites, Brevidorsa, Samarella, Coelonella, Knoxiella? with a few specimens of Eridoconchidae. Overall, the composition of ostracod assemblages is stable from the lower to middle subunits of the study section and points to shallow water environment on a carbonate deep shelf below the fair-weather wave base. The fauna shows relationships with Early to Late Devonian ostracods from South China, suggesting a narrow seaway between Sibumasu and South China terranes during Late Silurian to Early Devonian (late Ludlow to early Emsian).