Abstract Recent taxonomic works have recognized only two species of spiny pocket mice of the genus Heteromys (Rodentia: Heteromyidae) from Costa Rica. Within Costa Rica, the widespread H. desmarestianus is considered to occur throughout the wet Caribbean lowlands, as well as at middle and high elevations on the Caribbean and Pacific slopes of the country's main montane systems. In contrast, H. oresterus is known from only a few localities at high elevations in the western portion of the Cordillera de Talamanca in central Costa Rica. Our morphological and morphometric analyses of specimens from northwestern Costa Rica reveal the presence of an undescribed species of the genus, which we describe as Heteromys nubicolens. This new species ranges from 750 to 1840 m in elevation in the Cordillera de Tilaran and Cordillera de Guanacaste. Heteromys desmarestianus is found in the surrounding mesic lowlands and foothills. Externally, both species possess dark brown dorsal pelage, but H. nubicolens differs by overal...