Social media harassment, a cyberbullying behavior, poses a serious threat tousers and platform owners of social media. A growing body of research suggests involvingbystanders in interventions to combat deviant behaviors. In this paper, we contextualizethe bystander intervention framework and reporting literature to social media in order tounderstand why bystanders report social media harassment. Our contextualized inter-vention framework focuses on three sociotechnical aspects—the online social environment,characteristics of the technology platform, and their interplay—that explain bystanderreporting on social media platforms. We tested the model using data gathered from 291active Facebook users. We found that four contextualized factors, (1) perceived emergencyof the social media harassment incident, (2) perceived responsibility to report, (3) perceivedself-efficacy in using built-in reporting functions, and (4) perceived outcome effectivenessof built-in reporting functions for tackling social media harassment, shaped bystanders’willingness to intervene against social media harassment. In addition, we showed thatperceived anonymity of the reporting system counterbalances the negative influence of thepresence of others on bystanders’willingness to intervene. For research, we contribute tothe cyberbullying literature by offering a novel sociotechnical explanation of mechanismsthat shape bystanders’willingness to report social media harassment. For practice, we offerinsight into how to build safer and secure social media platforms for all users.