A single specimen of a small mouse from Mt. Banahaw–San Cristobal Natural Park, Quezon Province, Luzon Island, Philippines, is here described as a new genus and species. It is easily distinguished from all other murids by its small size (15 g), rusty orange fur, mystacial vibrissae that are two-thirds the length of head and body, postocular patch of bare skin with long vibrissae arising within it, long tail with elongated hairs only on the posterior quarter, ovate ears, procumbent incisors that are deeply notched at the tip, and other distinctive characters. Both morphological and molecular data (from two nuclear genes) indicate that the new taxon is a member of the endemic Philippine clade of “giant cloud rats,” some of which weigh up to 2.6 kg. It is most closely related to the genus Carpomys, which includes the smallest previously known member of the clade (ca. 125 g), but differs from it in many features. The discovery of this new taxon reveals an even greater degree of diversification within the giant cloud rat clade than recognized previously, and adds to the 21 previously known genera of mammals endemic to the Philippines. The new mouse was captured in regenerating lowland rain forest located only 80 kilometers from Manila. This discovery highlights the importance of protecting regenerating tropical lowland rain forest, as well as the few remaining tracts of old-growth lowland rain forest on Luzon.