The Naihai cutlassfish (Trichiurus nanhaiensis, Wang and Xu, 1992) is an important commercial fish species in South China Sea, while the taxonomy is still controversial. The 17,060 base pair mitochondrial genome sequence of T. nanhaiensis was determined, which shared the features with the other bony fishes for gene arrangement, nucleotide composition, and tRNA structures except for tRNA(pro) gene. The termination-associated sequence, central conserved sequence blocks (CSB-F, CSB-E, CSB-D), and conserved sequence blocks (CSB-1, CSB-2, CSB-3), as well as tandem repeats were detected in the control region. Phylogenetic analyses revealed that T. nanhaiensis together with T. lepturus, T. japonicus, and T. brevis formed a clade of Trichiurus based on the complete Cyt b genes and partial CO I genes; meanwhile, the closest relationship was established between T. nanhaiensis and T. lepturus.