The European willow Salix atrocinerea introduced to eastern North America is considered a distinct species—and not a subspecies of S. cinerea. Diagnostic characters distinguishing S. atrocinerea from related willows occurring in eastern Massachusetts are discussed. S. cinerea is present in Massachusetts along with S. atrocinerea, though it is less common. More evidence is needed in order to justify the inclusion of other willows from the same section, S. caprea and S. aurita, on the list of European plants naturalized in New England. Some of these records might be based on S. atrocinerea (or S. cinerea) hybrids with the native S. bebbiana or S. discolor. A possibility of hybridization between the alien S. atrocinerea and native S. humilis is demonstrated through obtaining hybrid seedlings from S. humilis seeds collected in the Boston area. S. atrocinerea is known to be invasive in some countries outside the United States. In New England, its hybridization with the native willows may create a threat for their existence in addition to displacement. S. bebbiana appears to have been eliminated from the vicinity of Boston, the area invaded by S. atrocinerea.