BACKGROUND: Transgender and gender diverse (TGD) adults experience high levels of stigma that contributes to elevated substance use and HIV sexual risk behaviors. Despite higher burdens of substance use and HIV compared to cisgender adults, TGD individuals may be less likely to engage in health care to avoid further discrimination. SETTING: This analysis included 529 TGD adults in Massachusetts and Rhode Island who were HIV negative or had an unknown HIV serostatus and were purposively sampled between March and August 2019. METHODS: We used structural equation modeling to test whether substance use, HIV sexual risk behaviors (ie, condom use, sex work, and multiple partners), and receiving gender-affirming hormone therapy mediate any observed association between TGD-related stigma and utilization of HIV prevention clinical services (ie, HIV prevention programs, PrEP use, and HIV testing). RESULTS: Substance use and HIV sexual risk mediated the relationship between TGD-related stigma and utilization of HIV prevention clinical services (β = 0.08; 95% CI = 0.05, 0.17; P = 0.03 and β = 0.26; 95% CI = 0.14 to 0.37; P < 0.001). Having a hormone therapy prescription was not a mediator between TGD-related stigma and HIV prevention clinical services. CONCLUSIONS: Future interventions that aim to improve HIV prevention clinical services among TGD adults should consider the impact of TGD-related stigma on participants substance use and sexual risk behaviors. These efforts require that health care organizations and community organizations make a deliberate investment in the reach and success of interventions and programs.