Tropidia namasiae C. K. Liao, T. P. Lin & M. S. Tang, a new species of Tropidia Lindl. (Orchidaceae, Epidendroideae, Tropidieae) from the Namasia District in southwestern Taiwan, is described and illustrated. The species is mainly characterized by its subopposite and ovate to lanceolate leaves and dorsal sepals (3.5–4 mm), the smallest observed among species of Tropidia in Taiwan. The new species is closely related to T. angustifolia C. L. Yeh & C. S. Leou, but differs in leaf shape, size, and number as well as the number of flowers per raceme. Morphological similarities between these two species can be seen in the shapes of the floral lip and column, as well as lengths of the leaf blades. A key to the six species of Tropidia in Taiwan is also provided.