Rhodobryum is a genus of robust mosses that have a strongly rosulate habit, subterranean stolons, a costa in cross-section that either lacks or has only a small cluster of dorsal stereids, and is usually polysetous. There has been considerable debate about whether or not the taxon should be recognized as a subgenus of Bryum or as a distinct genus. Bryum is a large and complex genus that displays a considerable amount of homoplasy. Within the genus there are species with rosulate habits (some exhibiting polysety) and a few species that are larger than many Rhodobryum species. The subterranean stolons of Rhodobryum are a distinctive feature of the genus, but they cannot always be found, and the morphological distinctions between Rhodobryum stolons and the primary stems of strongly verticillate Bryum species are not clear. The character that best distinguishes Rhodobryum is found in its costa. In nearly all species the costa lacks dorsal stereids. Indeed, this character in combination with those noted above appears to sufficiently separate it generically from Bryum. In preserving the spelling of  Rhodobryum (Isoviita & Koponen 1984) the genus has been conserved with R. roseum rather than R. leucocanthum Hampe (= R. aubertii (Schwaegr.) Thér.) as the type species. This is unfortunate because R. roseum has dorsal stereids in its costa, while R. leucocanthum lacks them. Although Iwatsuki & Koponen (1972) make a quantitative distinction between the dorsal stereids in R. roseum and those in a typical Bryum-type costa, their figures (Fig. 2 d,k,o,p, Fig. 4 k) as well as those of Mohamed (1984) are not convincingly different.

Bryum procerum (Besch.) Par. and Bryum huillense are often placed in Rhodobryum. Bryum procerum is a robust species with evenly foliate leaves that are often doubly serrate. It is usually polysetous. Bryum huillense is a medium-sized species with a distinctly verticillate habit and monosetous condition. These species are excluded from Rhodobryum because they lack stolons and have large clusters of dorsal stereids in their costae.