Objective: Although many autistic adults show high symptoms of posttraumatic stress, posttraumatic stress disorder (PTSD) is underdiagnosed in this population. This study aimed to examine correlates of autistic adults meeting PTSD criteria (PTSD+) as well as predictors of professional PTSD diagnosis (Diagnosis+) in the PTSD+ subgroup. Method: 677 self-identified autistic adults completed an online survey on their demographics, mental health symptoms, and trauma history. T-tests and chi-square were used to compare subgroups, and logistic regression was used to predict diagnosis status. Results: PTSD+ participants were less likely to be employed or identify as cisgender men; had more mental health symptoms and worse functional impairment; and had experienced a higher number of traumas and more interpersonal trauma. The same was true for Diagnosis+ participants, who were additionally older and more likely to be racial/ethnic minorities. Older age, being a woman or gender minority, being unemployed or on disability, increased posttraumatic stress, having more co-occurring conditions, lower anxiety, and lower functional impairment predicted being Diagnosis+ among participants who were PTSD+. The final model explained 35% of variance in diagnosis. Conclusion: PTSD is associated with significant impairment in autistic adults, but it often goes unrecognized. Autistic cisgender men in particular may be underdiagnosed with PTSD because of gendered stereotypes. High anxiety and functional impairment may also increase barriers to obtaining an appropriate diagnosis. Future research should include participant treatment history as a potential factor. Clinicians should be aware of these potential signs of PTSD and diagnostic barriers when working with autistic clients.