Podostemaceae show different patterns of morphological variation relative to molecular ones between genera and between species, but additional material was necessary to make the patterns clearer. Using new material collected from Cambodia, we compared the variations of Terniopsis chanthaburiensis and T. heterostaminata in Cambodia, Laos and Thailand, and conducted matK phylogenetic analysis with many samples and most species of the genus. In contrast to the narrow molecular variation, the morphological variation (e.g., in the length of the shoots and pedicels) is large and continuous. The results indicate that variation in the two species is intraspecific. A similar pattern exists in two pairs of other species of Terniopsis, in which the morphological variation is large and discontinuous, while there is little molecular difference. The opposite pattern is present in other cases (e.g., Dalzellia zeylanica and Tristicha trifaria). The variation in T. chanthaburiensis and T. heterostaminata does not appear to be a response to variation in the habitats, that is submerged rock surfaces in fast currents. Distributional and phylogenetic data indicate that Terniopsis diversified primarily in Laos and Thailand, then expanded into neighboring regions. A synopsis of the genus Terniopsis and its 14 species is given.