Little is known about Ordovician litho- and biostratigraphy in northern and western Thailand because outcrops are scarce in these mountainous areas. Biostratigraphic and biogeographic studies of conodonts are limited to the uppermost Floian to lower Darriwilian strata in western Thailand. This paper describes and discusses a new Middle Ordovician conodont fauna that includes nine species from the Thong Pha Phum area of western Thailand. This is the first report of Eoplacognathus suecicus, Pygodus lunnensis, Protopanderodus calceatus, Protopanderodus cooperi, and Protopanderodus graeai in Thailand and indicates that a cool-water fauna existed in western Thailand and in southern peninsular Thailand during the Middle to Late Ordovician. The Dapingian to early Darriwilian sea-level rise likely resulted in an immigration of this “cooler or deeper water” fauna to the Thong Pha Phum area.