The marine red alga Ahnfeltiopsis triquetrifolia sp. nov. (Phyllophoraceae, Gigartinales) is described from Japan. Foliaceous unisexual gametophytes are characterized by having three-edged axes, and crustose tetrasporophytes are distinguished by the following combination of features: tightly coalescent perithallial filaments, nemathecial sori, small numbers of both tetrasporangia (3 or 4) and sterile cap cells (1 or 2) in each fertile filament, and the absence of hypobasal tissue. Gonimoblast filaments develop from the auxiliary cell after it has fused with a carpogonium. Carposporangia develop from gonimoblast cells, from medullary cells contacted by the gonimoblast filaments, and from the cells produced by such medullary cells. Mature, compact cystocarps are immersed within the medullary layer and discharge carpospores through carpostomes formed in the thickened cortex. Cultured tetrasporophytes were induced to sporulate by transfer from a long-day to a short-day regime at 20°C, and one single short-day cycle was effective for tetrasporangial induction. Tetraspore germlings gave rise to basal discs from which upright axes developed. The upright axes showed a morphology similar to that of field-collected gametophytes, although they remained vegetative during 18 months in culture.