Two new marine gastrotrichs of the family Thaumastodermatidae are described from South Korea. Tetranchyroderma gracilium n. sp. bears pestle organs, tetrancres, and two pairs of large dorsolateral tubes. It seems most closely related to T. massiliense Swedmark, T. heterotubulatum Hummon, Todaro and Tongiorgi, and T. boreale Clausen. Thaumastoderma copiophorum n. sp. has five pairs of dorsal cirrata, the overall picture of which differs clearly from that of all known species. In the presence of accessory cirrata, it relates to T. appendiculatum Chang, Lee and Clausen.