We present the first county-level checklist of the bees of Massachusetts, including verified records of 390 species. We review the literature and historical material, and supplement these with recent collections and online image databases, compiling a dataset of over 100,000 records. Detailed accounts are provided for 50 species reported for the first time in Massachusetts, including six species reported for the first time in New England, and 49 other species noteworthy for their paucity of records, distributional significance, novel host/ parasite associations, or taxonomic uncertainty. The addition of newly reported species is largely the result of increased bee surveys in the past 15 years, including targeted sampling on known host plants. Twenty-three species represented in collections prior to 2005 are absent from recently collected material. The richness of the Massachusetts bee fauna is compared to that of neighboring states. Sixteen of the approximately 35 exotic species recorded from North America are verified from Massachusetts. We report recent rediscoveries in the state of Andrena rehni Viereck, 1907, and the regionally rare Epeoloides pilosulus (Cresson, 1878). Two new presumed host-parasite associations are made, those of Epeolus inornatus Onuferko, 2018 parasitizing the nests of Colletes banksi Swenk, 1908, and of Triepeolus obliteratus Graenicher, 1911 parasitizing the nests of Melissodes apicatus Lovell and Cockerell, 1906.