Abstract Public health officials have faced resistance in their efforts to promote mask-wearing to counter the spread of COVID-19. One approach to promoting behavior change is to alert people to the fact that a behavior is common (a descriptive norm). However, partisan differences in pandemic mitigation behavior mean that Americans may be especially (in)sensitive to information about behavioral norms depending on the party affiliation of the group in question. In July–August 2020, we tested the effects of providing information to respondents about how many Americans, co-partisans, or out-partisans report wearing masks regularly on both mask-wearing intentions and on the perceived effectiveness of masks. Learning that a majority of Americans report wearing masks regularly increases mask-wearing intentions and perceived effectiveness, though the effects of this information are not distinguishable from other treatments.