A species of large carpenter bee, Xylocopa virginica virginica (Linnaeus, 1771), is reported from Montana and Idaho, USA for the first time. This new Montana distribution record adds to a growing list of bee species found in the state and contributes to an ongoing inventory of the bees of Montana. The Montana collection likely does not represent an established population but a transient accidental introduction resulting from human transportation since Montana is considerably geographically separated from the published distribution of this species and no additional adults have been found. The Idaho collections may represent an established local population as there are several specimens and online records in or near Boise on multiple dates and years. An online record of a specimen of X. californica arizonensis Cresson, 1879 from Montana proved to be erroneous. The distribution of X. virginica is reviewed and the economic, social, and ecological implications of the occurrence or establishment of this species in Montana and Idaho are discussed.