Ranitomeya reticulata is one of the smaller species of poison dart frogs, hence its inclusion in the "thumbnail" species group. Males can reach approximately 12 mm in length from snout to vent, while the larger females may reach 15 or even 20 mm long. Like all poison dart frogs, R. reticulata are vividly coloured and patterned, which advertises their poison. Red-backed poison dart frogs have black legs with a cobalt or sky-blue mesh pattern, a black belly, and a back that ranges from fiery orange to scarlet in color, hence the common name. Like all arboreal frogs, R. reticulata possess suckerlike disks on their toes which makes their grip adhesive. As they are very small, they often attempt to advertise their poison by flaunting such colors or by ascending trees to escape from predators. If isolated from any form of escape, R. reticulata will use their poison as a defense mechanism. R. reticulata are more slimly built than many dendrobatids, which combined with their small size, gives them the ability to squeeze into minute hiding places.
