Background: In Brazil, ~35% of people living with HIV (PLHIV) have poor adherence to antiretroviral therapy (ART). HIV-related stigma is associated with worst continuum of care outcomes, however evidence from Brazil is scarce. We explored pathways between HIV-related stigma dimensions and ART adherence among Brazilian cisgender gay, bisexual and other men who have sex with men (MSM) living with HIV. Methods: A sample of MSM ≥18 years was recruited online between February/March 2020 through advertisements on Hornet, a location-based dating app. Validated scales were used to assess ART adherence and HIV-related stigma. Indirect and direct pathways between HIV-related stigma dimensions and ART adherence were estimated using structural equation models while considering socio-demographic and substance use related variables. Models were estimated using mean- and variance-adjusted weighted least squares, and goodness of fit indices were calculated. Findings: Among 1,719 MSM living with HIV who reported starting ART, 70% were adherent. There was evidence of indirect effects of concerns about public attitudes (standardized coefficient (SC) = -0.095, 95% confidence interval (95%CI) = -0.172 - -0.017) and personalized HIV-stigma (SC = -0.022, 95%CI = -0.043 - -0.001) on ART adherence mediated through negative self-image. Personalized HIV stigma and concerns about public attitudes were both positively associated with negative self-image (SC = 0.129, 95%CI = 0.066-0.193; SC = 0.549, 95%CI = 0.494-0.603), and concerns about public attitudes was associated with HIV disclosure concerns (SC = 0.522, 95%CI = 0.463-0.581). However, the direct paths from personalized HIV stigma and concerns about public attitudes to ART adherence were not significant. Interpretation: Our research underscores the critical need for multifaceted interventions to eliminate HIV-related stigma at both individual and societal levels. At the individual level, psychotherapeutic interventions to improve self-image might helpful. Additionally, public policy should aim to dismantle structural stigma with awareness campaigns on various media channels, integration of anti-stigma curriculum into schools, and training for professionals.