Objective: The objective of this study is to investigate the discriminative validity of neuropsychological tests and diagnostic assessment instruments in diagnosing adult ADHD in a clinical psychiatric population. Method: Of 108 patients, 60 were diagnosed with ADHD. The Diagnostic Interview for ADHD in adults (DIVA 2.0) and Adult ADHD Self-Report Scale (ASRS) v.1.1 together with eight neuropsychological tests were investigated. Results: All instruments showed poor discriminative ability except for the DIVA, which showed a relatively good ability to discriminate between the groups (sensitivity = 90.0; specificity = 72.9). A logistic regression analysis model with the DIVA and measures of inattention, impulsivity, and activity from continuous performance tests (CPTs) showed a sensitivity of 90.0 and a specificity of 83.3. Conclusion: Neuropsychological tests have a poor ability to discriminate between patients diagnosed with ADHD and patients not diagnosed with ADHD, but variables from CPT tests can contribute to increasing the specificity by 10% if used in combination with the DIVA.