In an effect known as hostile media perception (HMP), a perceptual bias believed to be driven by social identity processes, partisans tend to view objective news media as hostile towards their position. As advances in digital technology increase online news use, the inherent features and communicative properties of these technologies can influence how we express ourselves online and how we perceive online news content. This dissertation examined the impact of identity-related heuristics within social media comment on the HMP. Taking a social identity approach, a series of experiments tested the effects of audience position on the issue of arming teachers and perceived comment identity on the HMP. Audience position had a significant effect across studies as supporters of arming teachers reported greater HMP; evidence points to religiosity as a potential moderator of this relationship. Results for comment identity were mixed. Outgroup comments generally led to increased HMP; however, the difference was not significant across studies. There was also evidence of associations between outgroup comments and affective responses of defensiveness and negative emotion. The findings of this study offer support for the self-categorization explanation of the HMP, while contributing methodologically to the study of the HMP in the context of mobile social media. Results are discussed in light of literature on social identity and biased perception of online news.