ABSTRACT Forced displacement is on the rise, particularly in areas with high HIV prevalence. HIV transmission within and between forcibly displaced and local communities in situations of prolonged forced displacement can be facilitated by delayed HIV diagnosis and less viral suppression due to financial, logistical and administrative barriers placed on displaced people. Forcibly displaced men who have sex with men (fdMSM) can experience additional barriers due to stigmatization of their sexual orientation (MSM stigma), particularly in countries with high prevalence of MSM stigma such as Ukraine. Disruption of social networks can amplify the risks of substance use disorders (SUDs), fuel such stigma and enable HIV transmission. Ukraine has one of the largest HIV epidemics in Europe and is experiencing a military conflict in the eastern regions that resulted in displacement of >1.5 million Ukrainians. Consequently, fdMSM in Ukraine face unique challenges with respect to access to HIV prevention. We propose a longitudinal study of 1,200 MSM in Ukraine: 800 MSM (600 local and 200 fdMSM) in Kyiv, the capital of Ukraine, and 400 (300 local and 100 fdMSM) in Mariupol’, a town in Eastern Ukraine close to the conflict zone. Modified respondent-driven sampling will be used to recruit participants and collect social network and behavioral data. Blood samples will be collected for viral load testing, HIV genetic sequencing and subsequent phylogenetic analysis. All MSM living with HIV will be invited for follow-up interviews after 6- and 12- months post-baseline recruitment. Our overarching hypothesis is that fdMSM will have higher prevalence of SUDs and HIV and be more likely to appear in recent HIV transmission clusters compared to local MSM, and that fdMSM in Mariupol’ will have more stigmatizing social network ties that originated after migration compared to fdMSM in Kyiv. We will address the following Specific Aims: 1) Determine the prevalence of HIV, SUDs, MSM stigma, and HIV testing among local and fdMSM in Kyiv and Mariupil, Ukraine; 2) Characterize retrospective changes in social network composition since migration (since 2014) and the impact of these changes on SUDs, MSM stigma, and viral suppression; 3) Estimate the impact of MSM stigma and social support disruption on HIV transmission dynamics between and within local MSM and fdMSM using phylogenetic and phylogeographic analysis.