© 2020 Society for Adolescent Health and Medicine Purpose: HIV treatment as prevention is effective for reducing the risk of HIV transmission and the messaging campaign, undetectable = untransmittable, is gaining recognition. As youth living with HIV (YLWH) who have condomless sex may acquire and potentially transmit other sexually transmitted infections (STIs), the purpose of this study was to assess potential differences in transmission risk of HIV and other STIs among YLWH to inform subsequent HIV and STI prevention efforts. Methods: A cohort of 600 HIV behaviorally infected youth aged 13–24 years who were engaged in medical care completed an audio computer-assisted self-interview including questions about demographics, HIV disclosure, mental health, substance use, and sexual behaviors and beliefs. HIV viral loads and the presence of other STIs were abstracted from medical records. A viral load <200 copies/mL was considered undetectable. Univariate and bivariate analyses were conducted to examine differences by viral load and STIs. Results: Participants were categorized into four groups: (1) undetectable without STIs (55.2%); (2) undetectable with STIs (14.2%); (3) detectable without STIs (22.8%); and (4) detectable with STIs (7.8%). In comparison to the other three groups, youth in the undetectable group with STIs reported more favorable sexual risk reduction attitudes and beliefs, internet use for finding sex partners, anal sex with male partners, and condomless anal sex with male partners. Conclusions: YLWH with undetectable viral loads and other STIs engaged in higher risk behaviors. To realize the promise of the messaging campaign, undetectable = untransmittable, efforts must focus on sustained viral suppression and prevention of STIs among YLWH.