Objective: The prevalence of anxiety and depressive (i.e., internalizing) disorders is higher among bi+ individuals (i.e., individuals with attractions to more than one gender and/or who identify as bisexual or pansexual) compared to both heterosexual and lesbian/gay individuals. Cross-sectional research has demonstrated that stressors unique to bi+ individuals are associated with internalizing symptoms. However, longitudinal research examining these associations and underlying mechanisms is extremely limited. Method: We utilized five waves of data (6 months between waves) from a diverse sample of bi+ individuals assigned female at birth (age 16-32; 29% gender minority; 72.9% racial/ethnic minority) to examine: (a) prospective associations between three bi+ stressors (enacted, internalized, anticipated bi+ stigma) and internalizing symptoms; (b) potential mediating role of rumination in these associations; and (c) potential mediating roles of internalized and anticipated bi+ stigma in associations between enacted bi+ stigma and internalizing symptoms. Results: At the within-person level, when participants experienced more bi+ stressors than usual during a particular wave, they experienced subsequent increases in internalizing symptoms. Increases in rumination mediated these associations. Associations between enacted bi+ stigma and internalizing symptoms were mediated by increases in internalized and anticipated bi+ stigma. Conclusions: Findings indicate that bi+ stressors prospectively predict increases in internalizing symptoms and rumination may play a mechanistic role. Further, findings suggest that internalized and anticipated bi+ stigma may play mechanistic roles in the associations between enacted bi+ stigma and internalizing symptoms. Interventions targeting rumination, internalized stigma, and anticipated bi+ stigma may help to reduce internalizing symptoms among bi+ individuals. (PsycInfo Database Record (c) 2021 APA, all rights reserved).