Doxing on social networking sites (SNS doxing) has attracted scholarly and public attention due to the devastating consequences that this behavior can have on individuals and society. SNS doxing occurs when netizens disclose wrongdoers’ personal information on SNSs to punish them and hold them accountable for their unethical and improper acts. To offer a compelling theoretical explanation for this behavior beyond descriptive analyses of SNS doxing, we extend the social cognitive theory of moral thought and action by using retributive proportionality and social approval to articulate doxers’ moral judgments and decision-making. We conducted two independent studies using scenario-based and field surveys to validate the research model. The results of polynomial regression analysis suggest that users’ moral approval of SNS doxing is the result of the combined analyses between perceived blameworthiness of the doxee, perceived severity of SNS doxing, and perceived social approval of SNS doxing. We also found that users’ moral approval of SNS doxing determines their engagement in SNS doxing. Through this study, we offer rich theoretical and practical insights into the prevention of SNS doxing.