Few prospective studies have assessed whether antiretroviral therapy (ART) use is associated with changes in sexual risk behavior of human immunodeficiency virus (HIV)-infected persons in known HIV-serodiscordant partnerships. We conducted a longitudinal analysis of HIV-infected persons with known uninfected partners enrolled in the Partners Pre-Exposure Prophylaxis Study in Kenya and Uganda. Antiretroviral therapy use and self-reported sexual behavior were ascertained every 3 months. We assessed the effect of ART on sexual risk behaviors using zero-inflated negative binomial regression. Primary outcomes were condomless vaginal sex acts, pregnancy incidence and new sexually transmitted infection diagnoses. We followed 1817 HIV-infected persons (58% women) for 864 person-years before ART initiation and 771 person-years after ART. Median CD4 and plasma viral load at ART initiation were 277 cells/μL and 4.18 log10 copies/mL. Antiretroviral therapy use was associated with a significant decrease in condomless vaginal sex acts with HIV-uninfected partners (0.65 vs 0.39 per month; rate ratio, 0.64; 95% confidence interval [CI], 0.55-0.75; P < 0.001), but not condomless vaginal sex acts with nonprimary partners (1.30 vs 1.04 per month; rate ratio, 0.94; 95% CI, 0.94-1.20; P = 0.62). Pregnancy incidence was lower after ART (13.2 vs 8.4 per 100 person-years; HR, 0.71; 95% CI, 0.60-0.84; P < 0.001). Incident sexually transmitted infection diagnoses were similar (odds ratio, 1.05; 95% CI, 0.86-1.29; P = 0.63). Substantial risk compensation did not occur after ART initiation among East African HIV-infected persons with known HIV-uninfected partners. These data inform modelling studies of ART for HIV prevention by suggesting that risky sexual behavior did not appear to offset decreased HIV infectiousness in this key population.