We investigated the effect of incubation temperature on hatchling sex in the Japanese pond turtle, Mauremys japonica. Eggs obtained from females collected in the Tokai District of Japan invariably produced males when incubated at constant temperatures from 22.0 to 28.0 C, whereas they produced only females under a higher temperature of 30.0 C. Both males and females hatched from eggs incubated at 28.5, 29.0, and 29.5 C. These results indicate that M. japonica has a mechanism of temperature-dependent sex determination (TSD) with higher temperatures favoring females and lower temperatures males as in several other species of the family Geoemydidae, as well as of a few other testudine families so far studied. The pivotal temperature and the transitional range of temperature (TRT) in M. japonica obtained by the inverse estimation from logistic regression were 28.8 C and from 27.8–29.8 C, respectively. Possible effect of TSD on the sex ratio observed in some wild populations of this turtle is discussed.