Background High risk of substance use disorders (SUD) in people with ADHD calls for exploratory research. The aim of this study was to estimate the prevalence of alcohol use disorder (AUD) and drug use disorder (DUD) in a clinical sample of adults with ADHD, and to examine their association with ADHD symptom severity and emotional dysregulation (ED).Methods The study sample consisted of patients who were admitted to a private psychiatric outpatient clinic in Oslo between 2014 and 2018. Out of 612 patients diagnosed with ADHD, 585 (96.5%) agreed to participate in the study. ADHD was diagnosed according to DSM-5 criteria. AUD and DUD were diagnosed using the Mini International Neuropsychiatric Interview (M.I.N.I.). ADHD severity was assessed by the Adult ADHD Self Report Scale (ASRS). Emotional Dysregulation (ED) was assessed by the eight-item version of Barkley’s Current Behavior Scale - Self Report (CBS-SR).Results The 12-month prevalence of AUD and DUD was 5.3% and 13.7%, respectively. The lifetime prevalence was 12.0% for AUD and 27.7% for DUD. A history of DUD, but not AUD, was positively associated with hyperactivity-impulsivity ADHD core symptoms, as well as ED.Conclusions The prevalence of lifetime DUD among patients with ADHD is high and associated with higher levels of hyperactivity-impulsivity symptoms, as well as ED. It is important to consider comorbid DUD in adult ADHD patients, particularly among individuals with high levels of hyperactivity-impulsivity ADHD core symptoms or ED.