Stigmatizing social environments (of which 'structural stigma' is one component) negatively affect health-related outcomes. However, few studies have examined structural stigma related to sexual minority status as a risk factor for HIV outcomes among MSM. In August 2013, members of a large MSM social and sexual networking site in the United States completed a survey about HIV-prevention practices. A previously validated composite index provided values for state-level structural stigma, including density of same-sex couples, proportion of public high schools with Gay-Straight Alliances, state laws protecting sexual minorities, and public opinion toward homosexuality. Multivariable logistic generalized estimating equations assessed the relationship between structural stigma and condomless anal intercourse, use and awareness of antiretroviral-based HIV-prevention strategies (i.e. pre and postexposure prophylaxis, or PEP and PrEP), and comfort discussing male-male sex with primary care providers. Among the 4098 HIV-uninfected MSM, lower state-level structural stigma was associated with decreased odds of condomless anal intercourse [adjusted odds ratio (aOR) 0.97 per one unit increase in structural stigma score, 95% confidence interval (CI) 0.94-0.99], increased odds of awareness of PEP (aOR 1.06, 95% CI 1.02-1.09), and PrEP (aOR 1.06, 95% CI 1.02-1.10), having taken PEP (aOR 1.15, 95% CI 1.05-1.26) and PrEP (aOR 1.21, 95% CI 1.01-1.44), and comfort discussing male-male sex with providers (aOR 1.08, 95% CI 1.05-1.11), after adjusting for social and state-level confounders. MSM living in more stigmatizing environments had decreased use of antiretroviral-based HIV-prevention strategies compared to those in less stigmatizing environments. Legal reforms protecting sexual minorities should be evaluated as structural interventions that could reduce HIV risk among MSM.