Sexual minority women are more likely to experience negative mental health outcomes, including posttraumatic stress disorder (PTSD), than heterosexual women. Meyer’s (2003) minority-stress theory posits that this elevated risk of psychopathology results from stressors specific to the experience of holding a minority identity, and Hatzenbuehler (2009) suggested that these effects may be mediated by general psychological risk factors. Internalized heterosexism is conceptually and empirically linked to shame proneness, and both are associated with PTSD symptoms. Applying Meyer’s and Hatzenbuehler’s theories, in this study, we investigated the indirect effect of internalized heterosexism on PTSD symptomatology through shame proneness among 326 trauma-exposed sexual minority women recruited online. Analyses indicated that internalized heterosexism significantly and positively predicted PTSD symptomatology. In addition, internalized heterosexism had an indirect effect on PTSD symptoms through increased shame-related withdrawal tendencies, but not through shame-related negative self-evaluations. Clinical and policy considerations, including the need for systemic interventions focused on decreasing negative messages about sexual minority identities, as well as individual interventions targeting withdrawal behaviors in the treatment of PTSD among sexual minority women, are discussed.