Introduction: Persons with mild intellectual disability or borderline intellectual functioning (MID-BIF, IQ 50-85) are at high risk for the development of posttraumatic stress disorder (PTSD). A diagnostic instrument to establish a valid and reliable DSM-5 PTSD diagnosis in adults with MID-BIF was lacking. Aim of the current study was to determine the reliability and validity of the Adapted ADIS-C PTSD-adults for the assessment of PTSD according to DSM-IV-TR and DSM-5 in adults with MID-BIF. Method: 106 adults (18-72 years old) with MID-BIF were interviewed using the Adapted ADIS-C PTSD-adults. Results: Agreement between raters appeared to be good (mean Cohen's kappa for traumatic event scores 0.84, fulfillment of PTSD A-criterion 0.50, and PTSD symptom scores 0.90). Content validity was supported by a significant positive association with scores on the IES-IDs, a DSM-IV PTSD screening instrument (DSM-IV: r = 0.58; DSM-5: r = 0.43; ps <0.001). Convergent validity appeared to be good considering positive correlations between rates of PTSD symptoms and scores on the ADESS, measuring symptoms of anxiety and stress, depression, and social avoidance (DSM-IV: r = 0.47; DSM-5: r = 0.49; ps <0.001). Conclusion: The Adapted ADIS-C PTSD-adults is suitable for assessing DSM-5 PTSD in adults with MID-BIF, an important step to gain access to trauma-focused interventions that have shown to be applicable and potentially effective for this high-risk target group.