Growing to a carapace length of up to 16 inches (41 cm) and weighing up to 35 pounds (16kg), the radiated tortoise is considered to be one of the worlds most beautiful tortoises.

This tortoise has the basic "tortoise" body shape, which consists of the high-domed carapace, a blunt head, and elephantine feet. The legs, feet, and head are yellow except for a variably sized black patch on top of the head.

The carapace of the radiated tortoise is brilliantly marked with yellow lines radiating from the center of each dark plate of the shell, hence its name. This "star" pattern is more finely detailed and intricate than the normal pattern of other star-patterned tortoise species, such as G. elegans of India.

The radiated tortoise is also larger than G. elegans, and the scutes of the carapace are smooth, and not raised up into a bumpy, pyramidal shape as is commonly seen in the latter species. There is slight sexual dimorphism. Compared to females, male radiated tortoises usually have longer tails and the notch in the plastron beneath the tail is more noticeable.

Radiated tortoises occur naturally only in the extreme southern and southwestern part of the island of Madagascar and are a flagship species for the reptiles of this region. They have also been introduced to the nearby island of Reunion.

The radiated tortoise is a grazing herbivore. They feed during the day primarily on grasses, fruit and succulent plants, which form 80 to 90 percent of their diet. A favorite food in the wild is the Opuntia cactus. They are known to graze regularly in the same area, thus keeping the vegetation in that area closely trimmed. They seem to prefer new growth rather than mature growth because of the high-protein, low-fiber content.

Males first mate upon attaining lengths of about 12 inches (31 cm); females may need to be a few inches longer. The male begins this fairly noisy procedure by bobbing his head and smelling the females hind legs and cloaca. In some cases the male may lift the female up with the front edge of his shell to keep her from moving away.

The male then proceeds to mount the female from the rear while striking the anal region of his plastron against the female’s carapace. Hissing and grunting by the male during mating is common. Females lay from three to 12 eggs in a pre-excavated hole six to eight inches (15 to 20 cm) deep and then depart.

Incubation is quite long in this species, lasting usually between five and eight months. Juveniles are between 1.25 to 1.6 inches (3.2 to 4 cm) upon hatching. Unlike the yellow coloration of the adults, the juveniles are a white to an off-white shade. Juveniles attain the high-domed carapace soon after hatching.

They prefer dry regions of brush, thorn (Diderae) forests and woodlands of southern Madagascar. Radiated tortoises may live as long as 40 to 50 years.

Unfortunately, these tortoises are severely endangered due to loss of habitat, being poached for food, and being over exploited in the pet trade. It is listed in Appendix I of CITES, which prohibits the import or export of the species under most conditions. In 2008, they were elevated to Critically Endangered status with the IUCN. This species was once protected by local taboo but these have subsided and so radiated tortoises have been collected by the hundreds of thousands over the past 20 years.

There are 1405 specimens listed in the North American regional studbook and this population is managed by an SSP captive breeding program. Captive breeding to create an assurance colony for the future as well as helping the Madagascar people to conserve this species are some of the goals for the SSP.