Previous work has demonstrated the importance of social support and the deleterious effects of peer victimization on mental health among lesbian, gay, bisexual, trans, queer, and other sexual and gender minorities (LGBTQ+) youth. However, most studies on social support, victimization, and mental health have focused on samples with few trans youth and have examined general social support, rather than identity-based social support. In this study, we assess the relationships between identity-based social support, peer victimization, and mental health (anxiety, depression, and positive affect) in a sample of LGBTQ+ youth ages 12-17 (N = 351; M=14.4, SD=1.6; 70% White (non-Hispanic), 7% White (Hispanic), 6% Asian & White (non-Hispanic), 3% Black & White (non-Hispanic), 3% Native American & White (non-Hispanic), 2% Asian (non-Hispanic), and 9% Other/Unknown), in a majority trans youth (81%) sample. A subset of these youth completed the same measures twice (N = 142; M = 1.5 years between responses, SD=0.5 years; 87% trans), allowing us to assess these relationships longitudinally, as well. Consistent with our pre-registered hypotheses, cross-sectional analyses showed identity-based social support was associated with better mental health and identity-based victimization was associated with worse mental health. Unexpectedly, we did not observe a buffering effect: having identity-based social support did not reduce the association between identity-based victimization and negative mental health. We found mixed evidence about the relationships between changes in identity-based social support and victimization and changes in mental health. Overall, our findings are consistent with prior research in suggesting that identity-related support and identity-related victimization are important for LGBTQ+ youth's well-being.