A new species of terrestrial pitviper, Porthidium porrasi, is described from mesophytic forests of the Península de Osa and surrounding area of the Pacific versant of southwestern Costa Rica. It is most similar to P. nasutum and is characterized by a pattern of bands, persistence of the juvenile tail color in adults, and a high number of dorsal scales. Analysis of mtDNA sequences confirms its distinction from P. nasutum. The existence of this species reinforces the notion of elevated herpetofaunal endemism in southwestern Costa Rica.