Hypoderma diana Brauer, 1858, poses a significant threat to species of deer, yet records of this species in China remain fragmented and lack comprehensive documentation. Following high mortality in wapiti populations in Inner Mongolia during 2023–2024, surveys were conducted in the Gaogesitai Hanwula Nature Reserve in March 2025. Species identity was established through rigorous morphological analysis of third-instar larvae and an emerged adult female using light and scanning electron microscopy, supplemented by a 689 bp COI fragment. Results confirmed H. diana as the causative agent, with larvae localized along the host’s vertebral column. Comparative SEM analysis of the antennomaxillary sensory complex in H. diana, H. bovis (Linnaeus, 1758), and H. lineatum (Villers, 1789) revealed undifferentiated, volcano-like warts, contrasting sharply with the highly differentiated antennae and sensilla of Oestrus ovis Linnaeus, 1758. This study represents an inaugural record of H. diana in the Chifeng region and provides the first atlas of diagnostic features for both third-instar larvae and adults. These findings document H. diana as a critical agent of myiasis in the Manchurian wapiti, refining its known epidemiological profile in China and establishing an essential morphological reference for future wildlife health surveillance.