A new gastrotrich species of the genus Tetranchyroderma, bearing three-pronged cuticular hooks, was discovered in the Pacific, from littoral sandy bottoms of a beach at Joyagdo Island off Wando, south coast of the Korean Peninsula. Tetranchyroderma pinnatum, sp. nov., is characterized by feathered triancres, two pairs of rod-like cephalic tentacles, absence of dorsal and dorsolateral adhesive tubes, a short testis, and a large tube-shaped copulatory organ. Scanning electron microscopy is used to describe the fine details of the species, and a comparison among known species with triancres is provided. We also provide a key to the species of the genus Tetranchyroderma recorded in the northwest Pacific.