We present the first taxonomic revision of Selaginella subg. Ericetorum on the basis of an investigation of all known species, using optical microscopy and scanning electron microscopy. Selaginella subg. Ericetorum is distinguished from other Selaginella subgenera by the presence of the combination of monomorphic and decussately arranged leaves. Previous studies distinguished three species, namely, S. gracillima and S. uliginosa from Australia, and S. pygmaea from Australia and South Africa; our investigation revealed five species. Because of morphological differences and geographical separation, we proposed segregation of S. aboriginalis (of Australia) from S. pygmaea (of South Africa). The fifth species, S. royenii from Papua New Guinea, was described in 1956 but remained unnoticed. Although there are only five species, these are frequently misidentified in herbarium collections because of a lack of a monographic treatment. Therefore, in this study, we present two identification keys using different macro- and microstructural characters facilitating the identification of all the species. In addition, we present species descriptions and photographs showing structural details. For the first time, it is shown that the sculpture of Selaginella megaspores has sufficient diversity to distinguish closely related species while maintaining a high level of uniformity within a species.