Here, we present data on reproductive biology and sexual dimorphism of a species of Gymnophthalmus (G. vanzoi). We conducted the study in an urban area in the municipality of Santarém, on the campus of the Universidade Federal do Oeste do Pará, Pará state, Brazil. We collected data from 61 males, 79 females, and 9 individuals of unknown sex. Adults were sampled throughout the year, and juveniles were sampled for nine months. Adult females were larger and heavier than males. We observed sexual dimorphism in snout–vent length, length between limbs, head length, head width, head height, hind limb length, tail width, body mass, and coloration. We recorded reproductive females during 6 months, with the largest secondary follicles observed in June and August, the period of lowest precipitation. Reproductive males were recorded throughout the year, occurring at higher frequencies in December, January, and June–November. Testicular volume did not differ significantly among months (P = 0.0884). We collected the smallest individuals in January, March, July, August, November, and December, which suggests that recruitment occurs throughout the year. Despite the evidence of seasonality in the reproductive cycle with larger follicles and larger volume of testes in the drier season, we suggest that reproduction in G. vanzoi is continuous throughout the year, as we did not find statistical differences in the size of testicles (adjusted for subject body size) among months, and because recruitment occurs across various months of the year.