Antiretroviral therapy (ART) improves the health of people living with HIV and can reduce infectiousness, preventing HIV transmission. The potential preventive benefits of ART are undermined by beliefs that it is safe to have condomless sex when viral load is below levels of detection (infectiousness beliefs and risk perceptions). In this study we hypothesized that infectiousness beliefs and HIV transmission risk perceptions would prospectively predict people living with HIV engaging in more condomless sex with HIV-negative and unknown HIV status sex partners. Sexually active HIV-positive men (n=538, 76%) and women (n= 166, 24%) completed computerized interviews of sexually transmitted infections (STI) symptoms and diagnoses, unannounced pill counts for medication adherence, medical chart abstracted HIV viral load, and 28 daily cell-phone delivered prospective sexual behavior assessments. Results showed that a total of 313 (44%) participants had engaged in condomless sex with HIV-negative/unknown status sex partners and these individuals demonstrated higher rates of STI symptoms and diagnoses. Two-thirds of participants who had condomless sex with HIV-negative/unknown status partners had not disclosed their HIV status. Multivariable logistic regression models showed that beliefs regarding viral load and HIV infectiousness and perceptions of lower risk for HIV transmission resulting from HIV viral suppression predicted condomless sex with potentially uninfected partners over and above sex behaviors with HIV-positive partners and STI symptoms/diagnoses. Interventions that address HIV status disclosure and aggressively treat STI in sexually active people living with HIV should routinely accompany the use of HIV treatments as prevention.