Abstract Background PrEP uptake is low among non-Belgian men and transwomen who have sex with men, although the HIV epidemic among men who have sex with men in Belgium is diversifying in terms of nationalities and ethnicity. We lack an in-depth understanding of this gap. Methods We conducted a qualitative study using a grounded theory approach. The data consists of key informants interviews and in-depth interviews with migrant men or transwomen who have sex with men. Results We identified four underlying determinants which shape our participants’ experiences and contextualize the barriers to PrEP use. These include (1) the intersectional identities of being migrant and men and transwomen who have sex with men, (2) migration related stressors, (3) mental health and (4) socio-economic vulnerability. Identified barriers include: the accessibility of services; availability of information, social resources and providers’ attitudes. These barriers influence PrEP acceptance and mediated by individual agency this influences their PrEP uptake. Conclusion An interplay of several underlying determinants and barriers impacts on PrEP uptake among migrant men and transwomen who have sex with men, illustrating a social gradient in access to PrEP. We need equitable access to the full spectrum of HIV prevention and care for all priority populations, including undocumented migrants. We recommend social and structural conditions that foster exercising these rights, including adapting PrEP service delivery, mental health and social support.