Advances in HIV treatment have generated optimistic beliefs about HIV transmission risk and sexually transmitted infection vulnerability, which can influence motivation to practice safer sex. This study sought to better understand the relationships between these beliefs and different types of sexual risk behavior. A second objective was to determine the association between treatment adherence and sexual risk behaviors. Data collected from 842 HIV-positive men who have sex with men on treatment were examined. Eleven questions measured beliefs regarding HIV transmission risk and vulnerability to other infections or negative health outcomes. The main outcomes were concordant and discordant unprotected anal sex with main and non-main male partners. Thirty-four percent reported concordant and 41% reported discordant unprotected anal sex in the past three months: 26% reported discordant insertive unprotected anal sex. In all multivariate logistic regression models predicting sexual risk behaviors, at least one belief item remained statistically significant. Different patterns of associations between beliefs and sexual risk were observed across partner type and serostatus. Compared to transmission risk beliefs, more vulnerability beliefs were associated with sexual risk. Missing at least one treatment dose in the past month was associated with concordant and discordant sexual risk with non-main partners, while intentionally missing a dose was associated with only discordant risk with non-main partners. Post hoc moderator analysis explored potential interaction between beliefs and adherence. The belief that a low viral load lowers transmission risk was positively associated with discordant sex with non-main partners only among those who missed a dose intentionally. These results underscore the complex relationship between HIV transmission risk and vulnerability beliefs and sexual behavior with different types of partners. Prevention programs should carefully consider how to craft and tailor messages about medical advances while at the same time reinforcing the need for continued sexual safety.