Unfavourable social environments can negatively affect the health of gay, bisexual, and other men who have sex with men (MSM). We described how experienced homonegativity - negative perceptions and treatment that MSM encounter due to their sexual orientations - can increase HIV vulnerability among young MSM. Participants (n = 44) were young MSM diagnosed with HIV infection during January 2006-June 2009. All participants completed questionnaires that assessed experienced homonegativity and related factors (e.g. internalised homonegativity). We focus this analysis on qualitative interviews in which a subset of participants (n = 28) described factors that they perceived to have placed them at risk for HIV infection. Inductive content analysis identified themes within qualitative interviews, and we determined the prevalence of homonegativity and related factors using questionnaires. In qualitative interviews, participants reported that young MSM commonly experienced homonegativity. They described how homonegativity generated internalised homonegativity, HIV stigma, silence around homosexuality, and forced housing displacement. These factors could promote HIV risk. Homonegative experiences were more common among young Black (vs. non-Black) MSM who completed questionnaires. Results illustrate multiple pathways through which experienced homonegativity may increase HIV vulnerability among young MSM. Interventions that target homonegativity might help to reduce the burden of HIV within this population.