A new species of the murid Hydromys is named and described, based on skins, skulls and denti tions. The sample comes from the Wissel Lakes region in the Indonesian half of New Guinea (Irian Jaya). In many features, the new species is a miniature version of Hydromys chrysogaster and may be more closely related to that large water rat than to H. habbema. Nothing is known about habits or habitat of the new rat, and it has not yet been collected elsewhere in Irian Jaya or in the eastern half of New Guinea (Papua).