Species of Riella subgenus Trabutiella are characterized by their winged involucres. Although several species are included in this subgenus, to date only one, R. affinis, is monoicous. During the course of a worldwide revision of Riella including samples throughout the distribution area of R. affinis, populations with exceptionally different spore characters were discovered after examination of herbarium and fresh specimens and laboratory cultures. Morphological traits studied included spore characters measured by light and SEM microscopy. All samples had a high gametophytic similarity. Nonetheless, samples from Californian populations differed from the remaining ones of R. affinis in female involucre morphology, and more conspicuously in spore morphology and ornamentation. Statistical analyses of spore morphological traits confirmed the significant differences between the two species. Therefore, a new monoicous species of Riella subgen. Trabutiella is described as R. heliospora. This species can be distinguished from R. affinis by the female involucres with broader wings, the spores showing a triradiate mark on the proximal face, the presence of a conspicuous reticulum of basal membranes on the distal and proximal faces, longer and less dense spines on the distal face, and a conspicuous wing-like marginal webbing at the equatorial plane.