People living with HIV (PLWH) face difficult decisions about disclosing their HIV status to new sexual partners. Alcohol and other drug use could impact these decision-making processes and subsequent sexual risk behavior. We sought to examine the event-level relationships between substance use, HIV disclosure, and condom use in PLWH and their first-time HIV-negative or unknown status sexual partners. Adult PLWH were recruited from care settings in a southeastern U.S. city. Participants reported their sexual behavior for 28 consecutive days via text message prompts. We employed multilevel covariation in a causal system to examine the event-level relations between substance use and condom use. We proposed that this relationship would be mediated by HIV disclosure and moderated by viral suppression status. A total of 243 participants (83% male, 93% Black) reported 509 sexual events with first-time HIV-negative/unknown status sexual partners. Substance use at the time of sex was negatively associated with disclosure in PLWH with suppressed viral load (OR 0.29, β = - 1.22, 95% CI [- 2.42, - 0.03], p = .045), but differentially associated with condom use in PLWH with detectable versus undetectable viral load. In PLWH with viral suppression, participants who always disclosed versus who never disclosed their HIV status were more likely to use condoms (β = 1.84, 95% CI [0.35, 3.53], p = .017), but inconsistent disclosers were less likely to use a condom after disclosing (OR 0.22, 95% CI [0.07, 0.68], p = .008). Event-level analysis offers a more nuanced understanding of the proximal (substance use, HIV disclosure) and person-level (substance use, viral load) determinants of HIV transmission risk behavior in PLWH.