The New World species of the minute aquatic beetle genus Notomicrus Sharp compose a much greater diversity than their Old World congeners, with 14 of the 17 known Notomicrus species occurring in the Neotropics. A recent phylogenetic study recovered four primary New World species groups and found that there are a number of undescribed species across all of these main lineages. Here, we provide a taxonomic key to these New World species groups, including two described species that we currently do not place in any group (“ incertae sedis ” species), complete with images and illustrations of diagnostic characters and taxonomic notes including a list of known species in each group. This work provides a scaffold for further planned taxonomic revisions within the genus. In addition, we review the first of the four New World groups, the josiahi species group and describe one new taxon, N. interstinctus sp. nov. from northern Brazil. Provided are descriptions, habitus images and illustrations of diagnostic characters.