Ophiocordyceps sinensis (Ascomycota: Ophiocordycipitaceae) is a native fungal parasite of Hepialidae caterpillars and one of the most economically important medicinal caterpillar fungi in China. However, little is known about the phylogenetic and evolutionary relationships between O. sinensis and its host insects. In this study, nuclear ITS and ${eta}$-tubulin sequences from O. sinensis and mitochondrial COI, COII, and Cytb sequences from its hosts were analyzed across 33 populations sampled from five regions in China. Phylogenetically, both O. sinensis and its hosts were divided into three geographically correlated clades, and their phylogenies were congruent. Analysis of molecular variance and calculated coefficients of genetic differentiation revealed significant genetic divergence among the clades within both O. sinensis ($F_{ST}$= 0.878, $N_{ST}$=0.842) and its hosts ($F_{ST}$=0.861, $N_{ST}$=0.816). Estimated gene flow was very low for O. sinensis (Nm=0.04) and the host insects (Nm=0.04) among these three clades. Mantel tests demonstrated a significant correlation (P<0.01) between the genetic distances for O. sinensis and its hosts, as well as a significant association (P<0.05) between geographic and genetic distances in both. The similar phylogenetic relationships, geographic distributions, and genetic structure and differentiation between O. sinensis and its hosts imply that they have coevolved.