Weissia (90 species world-wide, Crosby et al. 2000) is a genus of fairly small, usually yellowish green plants with stems having a central strand and a hyalodermis. Its leaves are somewhat sheathing at base, with the upper and basal cells strongly differentiated, mid to upper leaf margins tightly involute, and a stoutly mucronate costa. The peristome in Weissia is typically reduced; in its best development there are 16 linear teeth, deeply cleft to a basal membrane, but more commonly the teeth are shorter and the basal membrane is rudimentary or absent. Most species of Weissia have densely pluripapillose leaf cells, however, there are some Neotropical species with leaf cells that are smooth dorsally and bulging-mammillose ventrally. The basal leaf  cells in Weissia are smooth and strongly differentiated from the upper cells with some species having the outer basal cells running up the margins in a v-shaped pattern.

The systematic positioning of Weissia is controversial. The genus is often placed in the subfamily Trichostomoideae near Trichostomum (Grout 1938, Andrews 1945, Saito 1975, Magill 1981, Zander 1994a). Both genera have a stem hyalodermis, stem central strand, and two well-developed stereid bands in their costae. In Weissia the peristome is often more reduced than that of  Trichostomum, and all its species have strongly involute leaf margins while those of Trichostomum are usually plane. However, there are Weissia species with fairly well-developed peristomes similar to those found in Trichostomum (e.g., W. jamaicensis and T. crispulum) and there are some Trichostomum species with strongly incurved upper leaf margins (e.g., T. crispulum). Both genera are similar to Tortella in having some species in which the outer basal leaf cells run up the margins in a v-shaped pattern. Spagnuolo et al. (1999), using ribosomal DNA analysis, provided further evidence for a close relationship between these three genera.

Zander (1993) offered an opposing view of the relationships of Weissia. In his view the presence in some species of Weissia of upper leaf cells that are dorsally smooth but bulging-mammillose ventrally indicate a relationship to the subfamily Pottioideae, tribe Hyophileae. Some members of this tribe also have a stem a hyalodermis and a central strand, as well as two well-developed stereid bands in their costae and a similar range of peristome form as seen in Weissia.