Objective: Functional near-infrared spectroscopy (fNIRS) provides direct and quantitative assessment of cortical hemodynamic response. It has been used to identify neurophysiological alterations in medication-naïve adults with attention-deficit/hyperactivity disorder (ADHD). Hence, this study aimed to distinguish both medication-naïve and medicated adults with ADHD from healthy controls (HC). Method: 75 HCs, 75 medication-naïve, and 45 medicated patients took part in this study. fNIRS signals during a verbal fluency task (VFT) were acquired using a 52-channel system and relative oxy-hemoglobin changes in the prefrontal cortex were quantified. Results: Prefrontal cortex hemodynamic response was lower in patients than HCs (p � �.001). Medication-naïve and medicated patients did not differ in hemodynamic response or symptom severity (p >.05). fNIRS measurements were not associated with any clinical variables (p >.05). 75.8 patients and 76 HCs were correctly classified using hemodynamic response. Conclusion: fNIRS may be a potential diagnostic tool for adult ADHD. These findings need to be replicated in larger validation studies. © ©The Author(s) 2023.