We describe morphological changes observed throughout the embryonic development of Mauremys japonica, a geoemydid species endemic to Japan. Eggs laid in captivity were incubated under two different temperature conditions, 26C and 30C. A total of 185 embryos and seven hatchlings were observed. We divided the whole embryonic development into 26 stages on the basis of morphological criteria previously proposed for Chelydra serpentina. Eggs of the turtle developed faster at 30C (taking 44–47 days for completion of the whole process) than at 26C (56–64 days). As has been reported previously, embryos of M. japonica developed to males after being incubated at 26C and to females at 30C, but without any visually recognizable differentiation in external morphology. Comparisons of embryos between M. japonica and three other testudinoid turtles revealed that the carapace of M. japonica started pigmentation at stage 18, i.e., earlier than the other three species by two stages.