We describe a new pitviper species, Bothrops alcatraz, of the Bothrops jararaca group, from Alcatrazes island, off the coast of Sao Paulo, southeastern Brazil. It differs from the mainland coastal populations of B. jararaca in southeastern Brazil mostly by its darker coloration; smaller size; lower number of ventrals, subcaudals, and infralabials; number and shape of anterior cephalic scales; shape of hemipenis spines; intense coagulant activity of venom; and three specific venom proteins. From Bothrops insularis, another island species from southeastern Brazil, the new species differs mainly by its color pattern, smaller size, lower number of subcaudals in males, and absence of hemiclitoris in females. Bothrops alcatraz presents some features that may be viewed as paedomorphic within the B. jararaca group, such as small adult size, proportionally large eyes, intense coagulant venom activity, and diet composed of centipedes and lizards. We postulate that the dwarfism and characteristics of venom in B. alcatraz may be related to its diet (similar to that of juveniles of the mainland B. jararaca). Bothrops alcatraz and B. insularis may have originated through the isolation of populations of a B. jararaca-like ancestor on the Alcatrazes and Queimada Grande islands, respectively. The new species is regarded as critically endangered due to its very small area of occurrence and the declining quality of its habitat.583303312