Rhaetus westwoodi Parry (Coleoptera: Lucanidae) is the type species of monotypic genus Rhaetus Parry, which is distributed only in the region of the Eastern Himalayas. Little is known about the biology, ecology, genetics, and conservation status of this rare beetle. In this study, the mitochondrial genome of R. westwoodi was sequenced for the first time to obtain useful genetic data for this monotypic genus. Like most of known mitogenome in beetles, the mitochondrial genome of R. westwoodi is a closed circular molecule 18,134 bp in length, comprising 13 protein-coding genes, 22 transfer RNA (tRNAs) genes, 2 ribosomal RNAs (rRNAs), and 1 control region. The nucleotide composition is 36.30% adenine (A), 31.20% thymine (T), 21.49% cytosine (C), and 11.01% glycine (G), with a highly biased AT content of 67.50%. The phylogenetic analysis showed that R. westwoodi and Hexarthrius vitalis Tsukamotoi were recovered as one clade with strong nodal support (1.00 Bayesian Phylogenetics and Phylogeography [BPP], 100% maximum likelihood bootstrap [MLB]). The topology also strongly supported that R. westwoodi appeared to be a sister species to Pseudorhaetus sinicus Boileau (1.00 BPP, 100% MLB).