We describe a new species of Microgramma (M. microsoroides) from the Atlantic Forest of Brazil, present a phylogeny of Microgramma, and provide a new generic circumscription. Microgramma comprises ca. 30 species in the Neotropics and perhaps two species in Africa and on islands in the Indian Ocean. Several species are contentious with regard to generic placement, and have been treated within Pleopeltis, Polypodium, and Solanopteris. From all known species of Microgramma, the new species differs by the sori forming irregular rows between adjacent primary veins. This finding is quite surprising, inasmuch as all other species in the genus have sori in two very strict rows, one row on each side of the midrib. The monophyly of Microgramma, as newly redefined, is strongly supported in chloroplast DNA-based phylogenetic analyses, using the genes rbcL and rps4, the rps4–trnS intergenic spacer, and the trnL–trnF intergenic spacer on 13 samples from 12 species. The new species is nested within Microgramma as sister to Microgramma megalophylla. Recognition of the relationships of M. microsoroides calls into question certain soral characters traditionally used to define genera of Polypodiaceae, as well as genera of ferns in other families. Sorus placement is notoriously variable in some genera and in many different families of Polypodiales.