Adult Radiated Tortoises usually show a distinct sexual dimorphism. In captivity, sexual maturity usually sets in earlier than in the wild due to consistent, good conditions. Females become sexually mature at 15 years of age, a body size of at least 24-26 cm and a weight of at least 5 kg. Males do not mature until 20 years of age, a body size of 30 cm or more, and a weight of over 8 kg. Males seem to reach sexual maturity much later than females. Presumably, they cannot successfully reproduce until they reach a certain size and have gained experience. Females do not cycle under unfavorable conditions, e.g., absence of seasons, or lack of light. Hormonal release and reabsorption of egg follicles in the body of female ray turtles is possible.

For many turtle keepers it is important to have an overview of the sex distribution in the population when assembling a breeding group. Since a clear expression of the sexual characteristics in Astrochelys radiata starts only late with sexual maturity, in captivity about 15 years, a sex determination of juvenile specimens on the basis of external characteristics is often very difficult and needs some experience and expertise to be able to make a reasonably safe prognosis about the sex. There are also specimens that show contradictory external sex characteristics. For example, there are females that have a concave plastron and males that have narrow anal shield angles and small tails.

Sometimes young females show clear beginnings of anal bulges around the cloaca early on. This sac-like and extremely stretchable tissue is more pronounced in females than in males. It probably serves to squeeze large eggs out of the cloaca more easily and without injury. Juvenile males can sometimes be stimulated to eject the penis from the cloaca by gently showering warm water over it. The tip of the penis is also sometimes briefly visible during relaxed bathing in warm water or urination. A bulged and erect penis of an adult male may be 2/3 of the total carapace length.