In the United States, the prevalence rates of HIV and sexually transmitted infections (STIs) are higher among individuals in community supervision programs (CSPs) than in the general population. However, to date, no couple-based HIV or STI prevention interventions have been implemented for the large number of men in CSPs. To determine the effectiveness of a 5-session couple-based prevention intervention, compared with a 1-session counseling, testing, and referral (CTR) program, in reducing HIV and STIs as well as condomless intercourse among men in CSPs and their female sexual partners. A randomized clinical trial was conducted from July 11, 2013 (first recruitment), through May 17, 2016 (last randomization). Participants were drug-involved men mandated to a CSP and their female sexual partners (n = 230 couples or 460 individuals). Participants were recruited from various CSP sites in New York, New York, and randomized into either the PACT (Protect and Connect) intervention condition or the HIV CTR control condition (n = 115 couples or 230 individuals in each arm). Analysis of behavioral outcomes used an intent-to-treat approach. Statistical analyses were conducted from November 1, 2017, through June 1, 2018. Self-reported data on sexual behaviors in the past 90 days were used to assess behavioral outcomes at all time points. Biomarkers were collected at baseline and 12 months, and behavioral outcomes were collected at baseline and 3, 6, and 12 months. A total of 230 couples (460 individuals) were included. The mean (SD) age of participants was 35.0 (12.8) years, and most participants (341 [74.1%]) self-identified as black or African American race/ethnicity. Of the 18 new cases of STIs identified at the 12-month assessment, 10 came from the PACT arm and 8 from the HIV CTR control arm. Compared with the control participants, PACT participants had 33% fewer acts of condomless vaginal and/or anal intercourse with their main partner (incidence rate ratio [IRR], 0.67; 95% CI, 0.45-0.99; P = .04), 70% fewer acts with other partners (IRR, 0.30; 95% CI, 0.12-0.74; P = .009), and 40% fewer acts with all sexual partners (IRR, 0.60; 95% CI, 0.42-0.85; P = .005) over the entire follow-up period. In addition, PACT participants were less likely to report being under the influence of drugs or alcohol the last time they had vaginal and/or anal intercourse with their study partners (odds ratio, 0.55; 95% CI, 0.31-0.96; P = .04) and had 26% fewer sexual partners in the past 90 days (IRR, 0.74; 95% CI, 0.61-0.88; P = .001). At 12 months, HIV and STI incidence did not differ significantly between the 2 arms. The PACT intervention appeared to reduce risky sexual behaviors, such as condomless intercourse; this finding suggests that a couple-based HIV and STI prevention intervention program may curb the burgeoning HIV epidemic in CSPs. ClinicalTrials.gov identifier: NCT01690494.