We discovered an unknown insect mining the leaves of Clermontia fauriei (Campanulaceae) in the Alakai Swamp, Kauai. Although the leaf mines superficially resembled those of the Hawaiian endemic genus Philodoria (Gracillariidae), or possibly Euperissus (Cosmopterigidae), rearing revealed an undescribed species of Carposina (Carposinidae). We describe it here as Carposina hahaiella sp. n., and include detailed information on the morphology, leaf mines, cytochrome c oxidase I sequences, and a parasitoid of the new species. Carposina hahaiella represents the first confirmed record of leaf mining in the fruitworm moth family, adding to a remarkable variety of larval habits in Carposinidae.