Failures in prior roll-out of HIV prevention efforts have widened disparities in HIV incidence by race/ethnicity among young sexual minoritized men (YSMM). We hypothesized greater perceptions of medical mistrust would be associated with lower willingness to get an HIV vaccine, mediating the relationship between race/ethnicity and willingness to accept a future HIV vaccine. HIV-negative and unknown-status YSMM 17-24 years old (n = 229) recruited via social media and men-for-men networking apps completed online surveys from September 2021 to March 2022. Participants were asked about demographics, medical mistrust (healthcare-related sexual orientation stigma, healthcare-related race stigma, global medical mistrust, and trust in healthcare providers), and willingness to accept a future HIV vaccine. Vaccine willingness was highest among White YSMM (96.0%) and lower among Black (71.0%), Latino (83.6%), and multiracial or another race/ethnicity YSMM (80.0%). Even after accounting for medical mistrust constructs as mediators, compared to White participants, Black participants had lower odds of being willing to accept a future HIV vaccine. Participants with greater trust in healthcare providers had higher odds of willingness to accept a future HIV vaccine. Gaps in willingness to get an HIV vaccine are evident among YSMM by race/ethnicity, indicating potential further widening of disparities in HIV incidence when a vaccine becomes available without intervention.