Phalacrognathus muelleri, colloquially known as the rainbow stag beetle, and also the king, magnificent or Mueller's stag beetle, is a species of beetle in the family Lucanidae. It is found in northern Queensland, Australia and New Guinea. It can come in red, green, black, and blue forms. It is the only species in its genus, Phalacrognathus, which is closely related to the genus Lamprima.