In the eastern Great Lakes region of North America, two tiger swallowtail species have previously been recognized, Papilio glaucus Linnaeus, 1758 and Papilio canadensis Rothschild & Jordan, 1906. A third entity, the Midsummer Tiger Swallowtail, has been treated as a P. glaucus × canadensis hybrid, and exhibits a mosaic of both intermediate and unique morphological and biological traits. Here we demonstrate that rather than being a localized, historically recent hybrid phenomenon, the Midsummer Tiger Swallowtail maintains its morphological and physiological distinctness over a large geographic region in the absence of one or both putative parental species, and was first documented in the literature nearly 150 years ago. Papilio solstitius sp. nov. is physiologically unique in delaying post-diapause development, which results in allochronic isolation between the spring flights of P. glaucus and P. canadensis, and the late summer flight of P. glaucus. Similarly, the geographic range of Papilio solstitius spans the region between the northern terminus of P. glaucus and southern limits of P. canadensis, remaining distinct in areas of sympatry. Defining the taxonomic identity of this unique evolutionary lineage provides an important baseline for further inquiry into what has served as an exemplary species group in evolutionary study.