Background: Ugandan fisherfolk are a priority population for the scale up of antiretroviral treatment (ART). However, a number of multi-level factors may pose a challenge to engaging this population in HIV care. Objectives: Our specific aims include: 1) examining the prevalence of substance use, and its effect on ART adherence; 2) exploring the role of norms of masculinity on men’s engagement in HIV care; and 3) identifying individual, interpersonal, normative environment, and physical/built environment-level determinants of HIV clinic attendance and ART adherence. Methods: A cross-sectional, mixed methods study design was employed with male fisherfolk on ART in Waksio District, Uganda. This study included a quantitative xiv structured questionnaire (N=300), and qualitative in-depth interviews (n=30). Aim 1 uses both the quantitative and qualitative data. Aim 2 uses only qualitative data. Aim 2 uses only quantitative data. Results: We identified sub-optimal engagement in HIV care among men overall. Aim 1 demonstrated alcohol’s negative effect on ART adherence, and alcohol’s greater effect on non-adherence among men taking twice daily regimens compared to once daily. Aim 2 demonstrated HIV as a threat to men’s masculinity. However, ART’s positive effects on health restored men’s ability to fulfill their masculine roles, which motivated men for continued engagement in HIV care. Aim 3 identified multi-level factors associated with missed HIV clinic visits, including those at the individual (age), interpersonal (marital status), normative (anticipated HIV stigma), and physical/built environment-levels (travel time to the clinic, structural-barriers to adherence, accessing care on a landing site). Factors associated with ART non-adherence included those at the individual (age, income) and normative levels (anticipated and enacted HIV stigma). Conclusions: These findings highlight an urgent need for strategies to improve clinic attendance and ART adherence among male fisherfolk on ART, and add to our understanding of the multi-level determinants affecting HIV care engagement among male fisherfolk. We discuss public health implications, and suggest a package of interventions that may be integrated into differentiated service delivery specific to this most-at-risk population, including brief screening and intervention for alcohol reduction, gender transformative programming, and the integration of peer support and stigma reduction into community-based platforms for ART delivery.