ABSTRACT Background Attention-Deficit/Hyperactivity Disorder (ADHD) constitutes a significant risk factor for the development of Substance Use Disorders (SUDs). Individuals with both conditions use more substances, and the pattern of substance use seems to differ between individuals with and without ADHD. Research aim and objectives To establish the prevalence of ADHD symptoms in adult South African treatment-seeking patients with SUD, and the pattern of substance use and presence of previous ADHD diagnoses and pharmacotherapy. Methods Adult patients (N = 360) were recruited from rehabilitation facilities and screened using the ADHD Self-Report Scale Symptom Checklist (ASRS-v1.1). Questions included demographics, substance consumption and current/historical use of ADHD-indicated medication. Results A third (36.0%) of patients with SUD screened positive for ADHD – 14.6% had been diagnosed with ADHD prior to admission, and indicated pharmacotherapy, while 68.5% of those screened positive were not hitherto diagnosed with ADHD (p < 0.001). A statistical difference was found for polysubstance use with the ADHD group significantly being treated more frequently for use of more than one substance (p = 0.04). Conclusions and implications High rates of untreated and unrecognized ADHD were found among treatment-seeking SUD patients. Preventative strategies are crucial to reduce substance use and the development of SUD in individuals affected by ADHD.