Sexually reproducing populations of the marine red alga Peyssonnelia rosenvingii Schmitz (Peyssonneliaceae, Gigartinales), for which only spermatangia and monosporangia, borne on the same thalli, have previously been known, are reported from the cold-temperate region of Japan. Plants are reproductive in late autumn and early winter. Each carpogonial branch consists of four or five cells, and the suprabasal cell of three- to five-celled auxiliary cell branches functions as an auxiliary cell. The absence of spermatangia and the presence of large, non-reproductive plants during the fertile season suggest that the spermatangial sori are ephemeral. After presumed fertilization a connecting filament develops from the hypogynous cell and runs in the horizontal plane. It branches irregularly and fuses with several auxiliary cells, forming several gonimoblasts along its length. Each gonimoblast produces five to eight carposporangial chains in conspicuously elevated nemathecia up to 350 μm high. Tetrasporangia are also formed in similar nemathecia up to 300 μm high and have three- to eight-celled pedicels.