Offline and online racial discrimination has been associated with mental health problems among adolescents of color. Pandemic shelter-at-home policies and the reignited racial justice movement increased the use of social media among youth of color, potentially exposing them to social media racial discrimination. Yet, it is unclear which aspects of social media significantly contributed to youth exposure to racial discrimination and associated mental health issues during this period. This study assessed the relationships among social media use (hours, racial intergroup contact, and racial justice civic engagement), individual and vicarious social media discrimination (defined as personally directed versus observing discrimination directed at others), and mental health among 115 black, 112 East/Southeast Asian, 79 Indigenous, and 101 Latinx adolescents (N = 407, 82.31% female, aged 15–18 years, M = 16.47, SD = 0.93). Structural equation modeling (SEM) analyses indicate that hours of use and racial justice civic engagement were associated with increased social media racial discrimination, depressive symptoms, anxiety, alcohol use disorder, and drug use problems. Furthermore, individual social media racial discrimination fully mediated the relationship between racial justice civic publication and depressive and alcohol use disorder. Vicarious social media racial discrimination fully mediated the relationship between racial justice activity coordination with depressive symptoms, anxiety, and alcohol use disorder. Alternative SEM models indicate that exposure to individual and vicarious social media racial discrimination increased depressive symptoms and drug use problems among youth of color, further increasing their social media use frequency and racial justice civic publication. The findings call for strategies to mitigate the effects of social media racial discrimination in ways that support adolescents’ racial justice civic engagement and mental health.