Hybridization induced by human activities, such as crossbreeding between invasive and native species, can adversely affect the natural biodiversity of an ecosystem. In Japan, the endemic turtle species Mauremys japonica is known to hybridize with the alien species Mauremys reevesii, and putative hybrids have been encountered in the wild. If M. japonica × M. reevesii hybrids can readily crossbreed with M. japonica, the hybridization with M. reevesii could lead to the extinction of pure M. japonica populations. However, information on the reproductive ability of M. japonica × M. reevesii hybrids is limited. In this study, we collected wild-caught hybrids from across western Japan to assess their reproductive ability. We investigated the nesting season timing, clutch size, embryonic development, hatching success, and sperm viability. The results showed that female hybrids nested during the same months as the parental species and had similar clutch sizes and hatching success. No embryonic development abnormalities were detected, and viable sperm were observed in all hybrid male semen samples. In conclusion, the fertility of M. japonica × M. reevesii hybrids appears to be similar to the fertilities of the parental species, posing a potential challenge for M. japonica conservation.