The Golden Lancehead (Bothrops insularis) is a critically endangered snake endemic to the Queimada Grande Island in southeastern Brazil. Here, we provide data on reproductive biology of B. insularis obtained in the field and from preserved museum specimens and compare our results with its mainland relative Bothrops jararaca. Similar to other Bothrops species, females of B. insularis attained larger body sizes than males, but the sexual size dimorphism (SSD) value is much lower than in B. jararaca. The seasonal timing of reproduction of B. insularis is similar to mainland B. jaracaca as well as other species of the genus Bothrops. Courtship observed in nature takes place in autumn and early winter (March to July); vitellogenesis occurs from summer to spring (March to December); ovulation and fertilization occur in early spring (September); embryonic development takes place in middle spring and summer (October to April); and recruitment of newborns is restricted to the summer (February andMarch). Both offspring size and offspring mass of B. insularis are similar to that observed in B. jararaca, but B. insularis produces a high frequency of atretic follicles. Apparently B. insularis has a lower reproductive frequency than B. jararaca, which is highly variable from year to year. The distinct reproductive traits of B. insularis (compared to B. jararaca) seem to be related to the ecological conditions of its insular environment, which may eventually be considered for the conservation management of this threatened species.