any of some 900 species of beetles (insect order Coleoptera) in which the mandibles (jaws) are greatly developed in the male and resemble the antlers of a stag. In many species the elaborately branched and toothed mandibles may be as long as the beetle itself. If handled carelessly, their pinch can draw blood from a person. In some cases, however, the mandibles are large enough to be a handicap to the beetle's movement.

Most stag beetles are robust and black or brown. However, a few are brightly coloured. Paintings of Chaisognathus granti from Chile, which is metallic green and iridescent red, and Phalacrognathus muelleri from Australia, which is metallic carmine and edged with green, have been used on postage stamps.

Adult stag beetles range between 8 and 40 millimetres (¹/₃ to 1 ²/₃ inches) in length, although the male of the East Indian Odontolabis alces is more than 100 mm long. The male giraffe stag beetle (Cladognathus giraffa) of India and Java is almost as long, and its jaws make up about half of its total length. Most stag beetles live around rotting logs on which the larvae feed. Adults feed on sap. Adults are attracted to lights at night.