The Japanese mamushi, Gloydius blomhoffii, is a medium-sized pit viper found in Japan, Korea, and China and is one of the most familiar snake species to Japanese people due to its nutritional and medicinal value, as well as its venomous nature. Although the reproductive biology (e.g., mating season, breeding cycle, spermatogenesis, follicular growth, and pregnancy period) of the mamushi has been partly investigated, there is no detailed report about how the embryos of the mamushi develop in the mother's oviduct. To provide new information about developmental and reproductive biology of the mamushi and to provide a basis for evolutionary developmental studies aimed at understanding morphological evolution of pit vipers, we described embryonic development of the mamushi covering the middle pharyngula and just before birth, during which pit viper-specific anatomical traits are formed.