Porthidium arcosae (SCHÄTTI & KRAMER, 1993) is a terrestrial, small-sized (ca. 720 mm total length), rare and little studied pitviper endemic to Ecuador, occurring in a variety of dry xeric forests, in the province of Manabí. Specimens preserved in collections and observations of snakes in captive conditions yielded new information about its natural history and potential distribution. The present update of our knowledge on the geographical and altitudinal range includes 12 localities in the province of Manabí. An ecological niche model predicted additional records along the coast of the provinces of Manabí (as far to the north as San Vicente, about 50 km beyond the known territory) and Santa Elena. Sexual dimorphism was not significant relative to the length of body and tail. However, there was a significant difference in the length-to-width ratio of the head, which is comparatively wider in females. Courtship and mating is briefly described from three captive groups. Porthidium arcosae feeds on lizards in its juvenile phase and rodents as an adult. Four more pitviper species are sympatric with P. arcosae: Bothrops asper, Bothrops punctatus, Bothriechis schlegelii and Porthidium nasutum. A case is reported of envenomation of a 29-year-old man from a bite by P. arcosae is reported. A conservation status assessment indicates that P. arcosae would classify for the IUCN category Endangered (EN).