Flies in the genus Rhagoletis (Diptera: Tephritidae) are economically important fruit pests, which also serve as models for studying modes of speciation and coevolution with their hymenopteran parasitoids. We describe the morphology of Rhagoletis bushi n. sp., and its geographic distribution, host association, phylogenetic relationships, and identify an associated species of parasitoid wasp. R. bushi n. sp. is in the tabellaria species group and infests the fruit of buffaloberry (Shepherdia argentea) in the Northern Great Plains of North America. There is a suite of morphological characters, and a unique host plant association, that are diagnostic of R. bushi n. sp. Further evidence for the validity of R. bushi n. sp. and its placement within the tabellaria species group comes from a multilocus molecular phylogeny for representatives of species in the tabellaria, pomonella, and cingulata groups inferred from five loci (COI, CAD, period, AATS, and 28S). Additionally, we report a species of parasitoid, Pachycrepoideus vindemmiae (Hymenoptera: Pteromalidae), attacking R. bushi n. sp.