Objective: Separation anxiety disorder (SAD) of adulthood was added to the group of anxiety disorders in the DSM-5. Attention-deficit/hyperactivity disorder (ADHD) displays elevated comorbidity with anxiety disorders. Aims of the present study are to determine a) the prevalence of SAD in a clinical, adult sample with ADHD and b) the correlates of SAD comorbidity in adults with ADHD. Methods: DSM-IV-Based Diagnostic and Evaluation Inventory for Adult ADD/ADHD to confirm ADD/ADHD diagnosis by the psychiatrist; Structured Clinical Interviews for the Symptoms of SAD (SCI-SAS); the Questionnaire for Adult SAD (A-SAQ); and the Inventory for Symptoms of SAD (SASI) were administered to ADHD patients and to the age, gender, marital status, and education-matched healthy controls. Results: Thirty adults with ADHD and 26 healthy controls were enrolled in the study. The ADHD group had significantly higher Adult Separation Anxiety Questionnaire (A-SAQ) and Separation Anxiety Symptom Inventory (SASI) scores than the control group. According to the Structured Clinical Interview for Separation Anxiety Symptoms (SCISAS), 53.3% of the ADHD group and 11.5% of the control group fulfilled the SAD criteria while 63.3% and 19.2%, respectively, fulfilled the childhood SAD criteria. Three patients with ADHD (10.0%) that did not fulfil the childhood SAD criteria had the diagnosis as adults, while six ADHD patients (20.0%) and four control patients (15.4%) had childhood SAD and recovered from it. Only one of the controls (3.9%) developed de novo SAD. We found signifycantly increased life-time, childhood, and adult prevalence of SAD in adult patients with ADHD. Conclusions: Due to the self-selected, clinical sample, this rate may not reflect the true population prevalence of SAD among Turkish adults; community studies evaluating SAD's prevalence are needed.