Extending theorization on bystander intervention, this study examined antecedents of upstanding, a communication practice in which bystanders communicate disapproval to norm violators, using the context of mask wearing during COVID-19. Survey findings from undergraduate students revealed that perceived legitimacy of intervention played a key role in decisions to stand up to mask norm violators. In addition, anticipation of hostile responses from the confronted party and bystander indifference discouraged upstanding behavior or intentions. Participants also expected to receive less support from bystanders in a hypothetical episode as an upstander than what they intended to offer an upstander as a bystander.