We examined the associations between perceived mental illness stigma and HIV risk and protective behaviors among adults with severe mental illness (SMI) in Rio de Janeiro, Brazil. We measured mental illness stigma across three domains ("Personal Experiences," "Perceived Attractiveness," and "Relationship Discrimination"), and examined the relationship between experiences of stigma in each domain and HIV risk and protective behaviors over the past 3 months in 98 outpatients with SMI. Those who reported greater "Relationship Discrimination" stigma were significantly more likely to be sexually active and to have unprotected sex; they were significantly less likely to report deliberately having fewer partners as a way to protect themselves from HIV. The role of stigma in unprotected sexual behavior should be examined further and considered in any HIV prevention intervention for people with SMI.