Using 55 wild-caught turtles of various sizes [straight carapace length (SCL): 35–227 mm], we estimated a time span of the subadult stage extending from the onset to completion of sexual maturation in female Mauremys reevesii from central Japan. We examined serum vitellogenin (VTG, a yolk-precursor protein) levels of the 55 turtles and thereby determined the size at the onset of sexual maturation. We also sacrificed nine of these turtles (SCL: 143–180 mm) and made anatomical observations focusing on their ovaries and oviducts in order to determine size at the completion of sexual maturation. Serum VTG was detected in all females with SCL ≥155 mm but not in any female with SCL <145 mm, thus indicating that the first vitellogenesis in M. reevesii occurs when SCL has reached around 150 mm. Of the nine turtles dissected, three (SCL: 172–180 mm) had oviductal eggs, corpora lutea, and/or enlarged follicles that would be ovulated in the next breeding season, while the other turtles (SCL: 143–168 mm) did not show any anatomical evidence of full maturity. It is therefore considered that the sexual maturation of female M. reevesii is complete when SCL has reached around 170 mm. A growth curve based on the age and SCL data of the wild-caught turtles indicated that SCLs at the onset and completion of sexual maturation (150 and 170 mm, respectively) corresponded to the ages of five and seven years, respectively. The time span of the subadult stage was therefore estimated to be two years in female M. reevesii.