Introduction: Comorbid attention deficit/hyperactivity disorder (ADHD) is present in 15–25% of all patients seeking treatment for substance use disorders (SUDs). Some studies suggest that comorbid ADHD increases clinical severity related to SUDs, other psychiatric comorbidities, and social impairment, but could not disentangle their respective influences. Objectives: To investigate whether comorbid adult ADHD in treatment-seeking SUD patients is associated with more severe clinical profiles in these domains assessed altogether. Methods: Treatment-seeking SUD patients from 8 countries (N = 1,294: 26% females, mean age 40 years [SD = 11 years]) were assessed for their history of DSM-IV ADHD, SUDs, and other psychiatric conditions and sociodemographic data. SUD patients with and without comorbid ADHD were compared on indicators of severity across 3 domains: addiction (number of SUD criteria and diagnoses), psychopathological complexity (mood disorders, borderline personality disorder, lifetime suicidal thoughts, or behavior), and social status (education level, occupational and marital status, and living arrangements). Regression models were built to account for confounders for each severity indicator. Results: Adult ADHD was present in 19% of the SUD patients. It was significantly associated with higher SUD severity, more frequent comorbid mood or borderline personality disorder, and less frequent “married” or “divorced” status, as compared with the absence of comorbid ADHD. ADHD comorbidity was independently associated with a higher number of dependence diagnoses (OR = 1.97) and more psychopathology (OR = 1.5), but not marital status. Conclusions: In treatment-seeking SUD patients, comorbid ADHD is associated with polysubstance dependence, psychopathological complexity, and social risks, which substantiates the clinical relevance of screening, diagnosing, and treating ADHD in patients with SUDs.