Background <p>Young men who have sex with men (MSM) and transgender women (TGW) in Southern Africa experience a disproportionate burden of HIV, compounded by stigma related to HIV status and sexual orientation or gender identity (SOGI). These stigmas undermine mental health and antiretroviral therapy (ART) adherence, yet few integrated, community-informed interventions exist in low-resource settings. This study assessed the feasibility, acceptability, and preliminary impact of a peer-led pilot intervention to reduce stigma, improve mental health, and strengthen ART adherence among young MSM and TGW living with HIV in Malawi, Zambia, and Zimbabwe.</p> Methods <p>A mixed-methods parallel design was used, combining quantitative self-administered questionnaires with qualitative in-depth interviews. Seventy-two participants aged 18–24 years were recruited through community-based organisations, with 70 completing baseline and endline assessments. The intervention was initially designed as an 8-week programme grounded in the information–motivation-behavioural skills (IMB) model, however, it was subsequently reduced to 4 weeks due to time and logistical constraints and was delivered by trained peer facilitators under clinical supervision. Quantitative outcomes included measures of HIV-related stigma, SOGI stigma, depressive symptoms (CES-D-10), alcohol use (AUDIT), and ART adherence self-efficacy. Descriptive statistics were generated, paired sample t-tests were used for continuous variables, and McNemar’s tests assessed changes in categorical outcomes. Qualitative data were analysed thematically to explore participant experiences and perceptions of the intervention.</p> Results <p>Statistically significant improvements were observed from baseline to endline, including reductions in internalised HIV stigma (<i>p</i> = 0.01), anticipated HIV stigma (<i>p</i> = 0.01), healthcare-related stigma (<i>p</i> &lt; 0.001), SOGI-related harassment (<i>p</i> = 0.02), and depressive symptoms (<i>p</i> = 0.01). ART adherence self-efficacy increased significantly (<i>p</i> &lt; 0.001). Although changes in sexual risk behaviours were not statistically significant, trends suggested increased condom use and reduced substance use before sex. Qualitative findings indicated enhanced self-acceptance, improved coping strategies (including reduced reliance on alcohol), increased confidence around disclosure, and strong acceptability of the peer-led, group-based intervention. However, some participants reported that sessions were delivered too quickly.</p> Conclusions <p>This pilot peer-led intervention was perceived feasible, acceptable, and showed promising improvements in stigma, mental health, and ART adherence self-efficacy among young MSM and TGW in three Southern African countries. The findings support the potential value of theory-informed, peer-delivered interventions to address HIV-related stigma and SOGI-related stigma and adherence challenges in highly marginalised populations, warranting further evaluation through larger and longer-term studies.</p>