Seventeen Pachybrachis species occurring in eastern Canada [Ontario (ON), Québec (QC), New Brunswick (NB), Nova Scotia (NS), and Prince Edward Island (PE)] are treated by the authors. Two new national records were discovered, both from southernmost Ontario: P. cephalicus Fall and P. luctuosus Suffrian. Four species were new provincial records: P. cephalicus (ON), P. luctuosus (ON, QC), P. obsoletus Suffrian (NB), P. peccans (PE). A fully illustrated key to the Pachybrachis of eastern Canada is provided and supported with extensive photographs, distribution maps, and plant associations.Three species were distributed from southern Ontario into at least one province in the Maritimes (P. nigricornis (Say), P. obsoletus Suffrian and P. peccans Suffrian). Six species were distributed along the shores of the Great Lakes (Erie, Michigan, and Ontario) and rivers (Ottawa, Saguenay and St. Lawrence), but unknown from central and northern ON and QC (P. bivittatus (Say), P. hepaticus hepaticus (F. E. Melsheimer), P. othonus othonus (Say), P. pectoralis (F. E. Melsheimer), P. spumarius Suffrian and P. trinotatus (F. E. Melsheimer)). Seven species were rare, five being found exclusively from southern ON (P. calcaratus Fall, P. cephalicus, P. luridus (Fabricius), P. subfasciatus (J. E. LeConte) and P. tridens (F. E. Melsheimer)), with two having, in addition, a disjunct population in QC (P. atomarius (F. E. Melsheimer) and P. luctuosus). One species was found to be the northern most extension of an eastern United States (US) distribution into the eastern townships of QC (P. m-nigrum (F. E. Melsheimer)). There were no Pachybrachis that could be considered arctic, subarctic, or boreal species; no specimens were found from Labrador and Newfoundland, and all species had southern affinities. Pachybrachis atomarius, P. calcaratus, P. luridus, P. subfaciatus, and P. tridens, not seen over the last 30–70 years, may be extirpated from eastern Canada.
