Objectives. To examine the associations between state-level structural cissexism and both mental health and unmet mental health care needs among a large national sample of gender-diverse adolescents (GDAs). Methods. We measured structural cissexism by using a composite index of state-level indicators, including antitransgender laws and policies and explicit cissexist attitudes. Using participants' state identifiers, we linked this measure to individual-level mental health data from a subsample of 5658 GDAs (aged 13-18 years; mean age = 15.8 years; SD = 1.5) from the LGBTQ National Teen Survey conducted in 2022. Results. GDAs in states with higher levels of structural cissexism reported greater depression and anxiety symptoms compared with those in states with lower levels. In addition, compared with GDAs in states with lower structural cissexism, those in states with higher structural cissexism were more likely to report unmet mental health care needs. Conclusions. Our findings demonstrate significant associations between structural cissexism and mental health and health care utilization among GDAs. These results underscore the urgent need for transgender-affirming and protective policies and multidimensional and multilevel interventions across social environments to support GDA well-being and ensure equitable access to mental health services. (Am J Public Health. 2026;116(2):240-250. https://doi.org/10.2105/AJPH.2025.308313).