Posttraumatic stress disorder (PTSD) symptoms may interfere with gay, bisexual and other men who have sex with men's (MSM) ability to engage in safe sex practices. An indirect relationship with dissociation may help to elucidate the relationship between PTSD symptom severity and condomless sex among MSM with childhood sexual abuse (CSA) histories. These relationships have not previously been examined in this group, which has a unique vulnerability for HIV acquisition. A cross-sectional sample of MSM with histories of CSA (N =290) was recruited at study sites in Boston, MA, and Miami, FL. Participants had a mean age of 37.95 years (SD =11.68), 22% were African American and 29.4% identified as Latino. The sample reported a mean of 10.47 (SD =4.38) lifetime PTSD symptoms and 26.4% met the clinical threshold for dissociation. Logistic regression models (adjusted for age, education, and substance use disorder) were used to assess indirect effects of dissociation on the relationship between lifetime PTSD symptoms and condomless anal/vaginal sex episodes with serodiscordant or unknown status partners in the past 3 months. Dissociation accounted for the association between lifetime PTSD symptom severity and condomless sex episodes. The Sobel test (Sobel = 2.04, p = .042; CI 95% bias-corrected bootstrap) suggested significant indirect effects for dissociation. Dissociation among MSM with CSA histories may compromise accurate appraisals of sexual risk and safety and increase vulnerability for HIV acquisition. Further research is warranted to address HIV prevention in the context of PTSD symptom severity to improve the mental health of MSM and increase the effectiveness of HIV prevention interventions.