This paper presents information on the morphological, morphometric, and dental sex differences in the dwarf round ray Urotrygon nana. We recorded 12 morphological traits, sex, the distribution pattern of dermal denticles, the number of tooth rows, and the tooth shape of 466 individuals. The disc width of females ranged from 50 to 172 mm and that of males ranged from 53 to 135 mm. A neuronal classification model and a correspondence analysis showed that female disc width was 21.5% broader, and the distance from the rostrum to the anus was 17.7% longer than that of males, whereas males presented 19.5% greater distance between the nostrils, 9.7% greater preorbital snout length, 6.8% greater cloaca to caudal‐fin length, 2.7% greater interorbital distance, and 1.1% greater total length than females. The disc of adult males, including the abdominal cavity area, was densely covered with dermal denticles, which were slightly larger than those observed in females. Females presented homodont dentition with molariform teeth and a smooth lozenge‐shaped crown with rounded margins. Males exhibited homodonty but with tooth morphology variations in individuals of different sizes (from molariforms to sharper cusp teeth). There were changes in disc shape (from subcircular to oval), distribution and size of dermal denticles (more abundant and larger), and tooth shape (from molariform to monocuspid teeth) during male development, from neonates to adults. U. nana exhibited sexual dimorphism in size, disc shape, number and shape of teeth, and distribution and size of dermal denticles. [ABSTRACT FROM AUTHOR]