The first record of the Darwin wasp, Cyrtorhyssa moellerii Bingham, 1898 (Hymenoptera, Ichneumonoidea, Rhyssinae) from Thailand is presented. Members of both sexes are fully described and illustrated. The biology of C. moellerii, a parasitoid of the pleasing fungus beetle Encaustes opaca Crotch, 1876 (Coleoptera, Erotylidae), is reported for the first time. Hosts were associated with standing deadwood of Anthoshorea henryana (Pierre ex Laness.) P. S. Ashton & J. Heck (Dipterocarpaceae) in dry evergreen forest, Nakhon Ratchasima province, northeastern Thailand. DNA barcodes (cytochrome c oxidase subunit 1 sequence (COI)) were generated for both host and parasitoid and phylogenetic trees constructed for these and other members of the same family and subfamily respectively. A key is provided to separate the three known species of Cyrtorhyssa. This is the first confirmed host record for a tropical species of Rhyssinae as well as the first from Erotylidae.