Genus-group taxon Rhombodineutus Ochs, 1926 is related to Paracyclous Ochs (Indonesia) and Callistodineutus Ochs (Melanesia), classified under Dineutus MacLeay. It occurs in New Guinea and New Britain Island, inhabiting streams and rivers primarily at low and medium altitudes where the species may occur abundantly. Most species live in the forests and have a fairly restricted range, while D. pectoralis Régimbart has passed a niche shift and adapted to exposed and exploited land and spread widely, splitting into a considerable number of races, some of which were found at an elevation of 1500-2000 m. The morphological characters are reviewed and their differentiation examined as a background for the classification of the taxon and a revision of its species which have been placed in three groups, containing 8 species and 11 subspecies. One new species and four new subspecies are described. Keys are provided to the various groups of taxa.