SCIENTIFIC ABSTRACT: Supervisory mistreatment has emerged as an important area of inquiry in the field of organizational psychology. Accumulated over the past 2 decades, there is growing evidence that the consequences of supervisory mistreatment extend beyond the individual victim to other organizational members, in particular, to third-party observers of the mistreatment. Extent theory and research on third-party reactions to mistreatment has yielded different predictions and results, which calls for an integration and synthesis of the literature. Integrating a social identity framework to the third-party literature, the current article proposes that third parties’ emotional and behavioral responses may vary as a function of their identification with the supervisor. In particular, we suggest that a third party’s perception of the fairness of supervisory mistreatment will be influenced by the extent to which they view their relationship with their supervisor as a defining part of their identity. When observed mistreatment is severe and third parties cannot rationalize the supervisor’s wrongdoing, they experience a relational identity threat which, in turn, shapes their reactions to supervisory mistreatment. We hope this article will provide an initial framework that will spark future research examining third-party reactions to mistreatment through the lens of identity theory.