Hexarthrius mandibularis is a species of large stag beetle. It belongs to the tribe Lucanini, in the family Lucanidae. This beetle has a dull, dark, brick reddish body. Both the legs and antennae are blackish. The most notable characteristic are the large, antler-like mandibles. These have small teeth running down the inside edge, with one large, forward-pointing pair located approximately a third of the way down from the tips. The tips are forked inward somewhat.