Bystanders Join in Cyberbullying on Social Networking Sites: The Deindividuation and Moral Disengagement Perspectives Cyberbullying on social networking sites escalates when bystanders join in the bullying. Bystanders’ joining-in behaviors reinforce the abuse, expose victims to a larger audience, and encourage further abuse by signaling their approval of the aggressive behavior. This study developed an integrative model that explains bystanders’ joining-in cyberbullying behaviors on SNSs to offer actionable insights into reducing such harmful behaviors. We tested the model using 1,179 responses using a scenario survey study. Our findings suggest that IT artifacts (including digital profile, search and privacy, relational ties, and network transparency) activated two key mechanisms that lead to cyberbullying joining-in behaviors: (i) the deindividuation experiences that attenuate self-identity and put salience on group/social identity, and (ii) the moral disengagement practices that permit the exercise of cognitive maneuvers to justify group-interested choices that do not align with social standard. The findings explain why people who do not know each other gang up to bully a target on social media. Platform owners who wish to discourage bystanders from joining in undesirable activities may consider regulating how users could share and access digital resources in a social network and should acknowledge the influence of social identity in igniting, driving, and prolonging harmful online group behaviors. Cyberbullying on social networking sites (SNSs) escalates when bystanders join in the bullying. Although researchers have recognized the harmful consequences of joining in cyberbullying behaviors, little is known about the role of information technology (IT) and its underlying mechanisms in fueling such negative group behavior on SNSs. To address this research gap, we develop and test an integrative model that explains bystanders’ joining-in cyberbullying behaviors on SNSs . Based on the theoretical premises of the social identity model of deindividuation effects (the SIDE model), we derive two deindividuation experiences enabled by SNSs, namely experienced anonymity and experienced social identity . We further use the social network research framework to gain insights into how IT features (i.e., digital profile, search and privacy, relational ties, and network transparency) enable these two deindividuation experiences. Considering the socially undesirable nature of joining-in behaviors, we integrate the SIDE model with moral disengagement theory to explain how deindividuation experiences allow bystanders to bypass their psychological discomfort when engaging in such behaviors through the practice of moral disengagement mechanisms. Our research model is tested using a scenario survey, with two samples recruited from Amazon Mechanical Turk and Facebook. Our results support the influences of IT-enabled deindividuation experiences on bystanders joining in cyberbullying and demonstrate the mediating effects of moral disengagement mechanisms in bridging the effects of deindividuation experiences on joining-in behaviors. For researchers, the integrative view offers a conceptual bridge connecting IT features, deindividuation, moral disengagement, and negative online group behaviors on SNSs. For practitioners, our findings provide platform owners and governmental agencies with directions on how to mitigate cyberbullying on SNSs and other forms of deviant and undesirable online group behaviors. History: Yulin Fang, Senior Editor; J.J. Po-An Hsieh, Associate Editor. Funding: This work was substantially supported by a Senior Research Fellow Scheme Award, Research Grant Council of the Hong Kong Special Administrative Region, China Grant HKBU SRFS2021-2H03; and partially supported by grants from the General Research Fund, Research Grant Council of the Hong Kong Special Administrative Region, China Grants HKBU12511016 and HKBU12500020. Supplemental Material: The online appendices are available at https://doi.org/10.1287/isre.2022.1161 .