Sexual minority women (SMW) are at high risk of trauma exposure and, subsequently, the development of posttraumatic stress disorder (PTSD). The authors extended a theoretical model explaining the higher risk of mental disorders in minority populations to the maintenance and exacerbation of PTSD symptoms among young adult SMW specifically. This study used observational longitudinal data from a sample of 348 trauma-exposed 18- to 25-year-old individuals assigned female sex at birth who identified as either bisexual (60.1%) or lesbian (39.9%) and met screening criteria for PTSD. Participants identified as White (82.8%), Hispanic/Latina (12.4%), American Indian/Alaska Native (13.5%), Black/African American (13.8%), and/or Asian/Asian American (4.9%). The authors investigated whether distal stressors (i.e., criterion A traumatic events, daily experiences of heterosexism) produced proximal stressors (i.e., trauma-related cognitions, internalized heterosexism) that maintained or exacerbated PTSD symptoms. Findings indicated that daily heterosexism longitudinally predicted trauma-related cognitions (i.e., cognitions related to the self, world, and self-blame). Internalized heterosexism and cognitions about the self longitudinally predicted PTSD symptom severity. In addition, a significant indirect effect was identified between daily heterosexism and PTSD symptoms via self-related posttraumatic cognitions. These findings suggest that exposure to minority-specific distal stressors appears to promote nonminority-specific cognitive processes that, in turn, may maintain or exacerbate PTSD among young adult SMW exposed to trauma. Clinicians should consider addressing daily heterosexism in young adult SMW presenting with PTSD and evaluate how these experiences might promote clients’ global, negative views regarding themselves.