By means of an integrative taxonomic approach using morphology and DNA barcodes, multiple cryptic species within Lasioglossum (Dialictus) petrellum (Cockerell) (Hymenoptera: Halictidae) were discovered. Analysis of DNA barcode sequence data from across the supposed range of L. petrellum revealed distinct genetic differences that correlate with patterns of morphological variation and geographical distribution. The most morphologically distinct pair of species had the smallest DNA-barcode gap. The taxonomic limits of L. petrellum are revised and four new species are described and illustrated: L. (D.) tuolumnense sp. nov., L. (D.) griswoldi sp. nov., L. (D.) droegei sp. nov., and L. (D.) viridipetrellum sp. nov. A key to species of the “L. petrellum” group is provided.