The marine red alga Polyopes tosaensis Kawaguchi & Masuda, sp. nov. (Halymeniaceae, Cryptonemiales) is described from southern Japan. It is distinguished from other members of the genus by the following combination of morphological features: (1) dichotomously or subdichotomously divided, short (2.0 – 3.6 cm in height) blades with segments 2 – 3 mm (occasionally 5 – 8 mm) in width; (2) the rare presence of surface proliferations; (3) the production of reproductive structures over the middle to the upper portions of the blade; and (4) the absence of tetrasporangial nemathecia. This morphological distinction of P. tosaensis is supported by molecular phylogenetic analyses using rbcL gene sequences. Polyopes tosaensis is positioned close to P. hakalauensis (Tilden) Abbott from Hawaii rather than four Japanese species of the genus, P. affinis (Harvey) Kawaguchi & Wang, P. lancifolius (Harvey) Kawaguchi & Wang, P. polyideoides Okamura and P. prolifer (Hariot) Kawaguchi & Wang. The pairwise distances between P. tosaensis and other species of Polyopes range from 10 – 94 bp (0.8 – 7.5%) and are similar to those between species of the large genus Grateloupia [13 – 111 bp (1.0 – 8.8%)] in the Halymeniaceae.