Morphological investigations involving nearly all available museum material representing New Guinea “moss-mice” (rodents traditionally classified in the genera Pseudohydromys, Neohydromys, Mayermys, and Microhydromys) reveal outstanding undiagnosed taxic diversity (a minimum of 16 species, versus the eight species previously described) and allow for redefinition of generic boundaries among these little-studied rodents. Apart from Microhydromys Tate and Archbold, 1941 (comprising two species, as recently revised by Helgen et al., in press), herein we recognize two genera of New Guinea moss-mice: Pseudohydromys Rümmler, 1934 (now incorporating Neohydromys Laurie, 1952, Mayermys Laurie and Hill, 1954, and “Microhydromys” musseri Flannery, 1989) and a newly described genus, Mirzamys. Species of Pseudohydromys are recorded from montane areas throughout New Guinea (elevations spanning 600 to at least 3800 meters), including the mountain ranges of the Central Cordillera, the Huon Peninsula, and the North Coastal ranges. We diagnose and review 12 species of Pseudohydromys, including six species described as new. The new genus Mirzamys is erected to accommodate two newly described species of small terrestrial rodents from middle and upper montane forests and subalpine grassland edges (1900–3450 m) in the mountains of central New Guinea. Together these two new species represent a distinctive hydromyin lineage that resembles the terrestrial New Guinea hydromyin genera Pseudohydromys and Paraleptomys in various traits. Ecological attributes of all recognized moss-mice taxa, both previously and newly described, are reviewed in light of all information currently available about their biology.