Objective: Diagnostic assessment of ADHD is challenging due to comorbid psychopathologies and symptoms overlapping with other psychiatric disorders. In this study, we investigate if a distinct pattern of neuromuscular dysregulation previously reported in ADHD, can help identifying ADHD in psychiatric patients with diverse and complex symptoms. Method: We explored the impact of neuromuscular dysregulation, as measured by The Motor Function Neurologic Assessment (MFNU), on the likelihood of being diagnosed with ADHD, affective disorder, anxiety disorder, or personality disorder among adults (n = 115) referred to a psychiatric outpatient clinic. Results: Logistic regression revealed that neuromuscular dysregulation was significantly associated with ADHD diagnosis only (OR 1.15, p < 0.01), and not with affective-, anxiety-, or personality disorders. Sensitivity and specificity for ADHD at different MFNU scores is provided. Conclusions: A test of neuromuscular dysregulation may promote diagnostic accuracy in differentiating ADHD from other psychiatric disorders in patients with an overlapping symptom picture. This may have important implications for clinical practice. More studies are needed.