The Atlas moth is one of the largest lepidopterans, with a wingspan measuring up to 24 cm (9.4 in) and a wing surface area of about 160 cm2 (≈25 in2). It is only surpassed in wingspan by the white witch (Thysania agrippina) and Attacus caesar, and in wing surface area by the Hercules moth (Coscinocera hercules). As in most silk moths, females are noticeably larger and heavier than males, while males have broader antennae.
Atlas moths lay a number of spherical eggs, 2.5 mm (0.098 in) in diameter, on the undersides of the leaves of food plants. After approximately two weeks, dusty-green caterpillars hatch and feed on their egg-shell, and then the foliage of citrus, cinnamon, guava, and evergreen trees. The caterpillars can grow to 11.5 cm (4.5 in) in length and 2.5 cm (0.98 in) in thickness. They have white, waxy, fleshy spines along their backs, which become more prominent at later instars. On the last abdominal segment beside the prolegs, there is a large green spot surrounded by an orange ring.
After reaching a length of about 11.5 cm (4.5 in), the caterpillars pupate. They spin a 7–8 cm long papery cocoon interwoven with desiccated leaves and attach it to a twig using a strand of silk. The adult moths emerge from the cocoon after approximately four weeks depending on environmental factors.
Adult Atlas moths are weak, unsteady fliers. To conserve energy, the moths rest during the day and fly at night. As they lack fully formed mouthparts, the adults cannot eat, subsisting entirely on fat reserves accumulated during the larval stage. As a result, they live for only a few days during which their sole objective is seeking out a mate. Adults may be found on wing throughout the year but are most abundant between November and January.
Females release pheromones through a gland on the end of the abdomen to attract a mate. Females stay near discarded cocoons, seeking out a perch where the air currents will best carry their pheromones. Males can detect and home in on these pheromones from several kilometers away using chemoreceptors located on their feathery antennae.
The body is small compared to the wings. The upper side of the wings is reddish brown with a pattern of black, white, pink, and purple lines. There are triangular, scale-less windows bordered in black on each of the four wings. The undersides of the wings are paler. The tips of both forewings have prominent extensions that resemble the head of a snake. The resemblance which is exaggerated by movements of the wings when the moth is confronted by potential predators.