The negative impact of discrimination on adolescent mental health is well established, but less is known about the effects of identity-based bullying (i.e., verbal or physical assaults targeting identity(ies)). The current study examined the impact of identity-based victimization (i.e., everyday discrimination and identity-based bullying) on mental health, and the protective role of teacher-student relationships, in a diverse sample of adolescents. Data from a diverse sample of 9th-12th graders (N = 986, 51% youth of Color, 52% cisgender girls, 22% sexual minorities, 41% free/reduced-price lunch status) in a semi-urban high school were analyzed using structural equation modeling analyses, including moderation and multi-group comparisons. Findings indicated that identity-based victimization is pervasive and negatively associated with mental health. Adolescents with stigmatized identities across sexual orientation, race, and gender faced a higher risk of experiencing identity-based victimization, and mental health challenges. Teacher-student relationships that were positive and autonomy-enhancing had a moderating effect on the association between identity-based victimization and mental health for many adolescents, such that they were not protective for those experiencing more severe victimization (i.e., high discrimination or identity-based bullying). Multigroup analyses indicated significant model fit differences across race and gender. Results suggest that extant school-based supports are not enough to combat the pervasive effects of identity-based victimization. Findings support the examination and implementation of changes in clinical and school-based practice and policy to better support stigmatized and victimized adolescents.