Checklists provide information about the species found in a defined region and serve as baselines for detecting species range expansions, contractions, or introductions. Bees are a diverse and important group of insect pollinators. Although some bee populations are declining, these patterns are difficult to document and generalize due to a lack of long-term studies for most localities. Documenting the diversity of wild bee communities is critical for assessing pollination services, community ecology, and geographical and temporal changes in distribution and density. Here, an updated checklist of the bees of the Commonwealth of Pennsylvania, USA, is presented. Since the first checklist was published (2010; 372 species), thousands of additional specimens from the state have been collected and databased, new species have been described in the region, and the taxonomic status of some species have changed. Specimen data from insect collections, databases, scientific literature, and unpublished records were compared to the original checklist. Seventy-nine new state species records – including 49 first-time reports – representing five of the six bee families in North America, were documented resulting in a total of at least 437 bee species reported from Pennsylvania. We highlight new county records and species persistence details. Our list includes a total of 23 exotic species and at least five species of conservation concern. Lists of species excluded from the state checklist and species anticipated to occur in Pennsylvania are also included. This checklist provides baseline data for researchers and the public. The benefits of insect collections, specimen databases, determination and voucher labels, and georeferencing to biodiversity studies and other aspects of biological research are also discussed.