Anger has high prevalence in clinical and forensic settings, and it is associated with aggressive behavior and ward atmosphere on psychiatric units. Dysregulated anger is a clinical problem in Danish mental health care systems, but no anger assessment instruments have been validated in Danish. Because the Novaco Anger Scale and Provocation Inventory (NAS-PI) has been extensively validated with different clinical populations and lends itself to clinical case formulation, it was selected for translation and evaluation in the present multistudy project. Psychometric properties of the NAS-PI were investigated with samples of 477 nonclinical, 250 clinical, 167 male prisoner, and 64 male forensic participants. Anger prevalence and its relationship with other anger measures, anxiety/depression, and aggression were examined. NAS-PI was found to have high reliability, concurrent validity, and discriminant validity, and its scores discriminated the samples. High scores in the offender group demonstrated the feasibility of obtaining self-report assessments of anger with this population. Retrospective and prospective validity of the NAS were tested with the forensic patient sample regarding physically aggressive behavior in hospital. Regression analyses showed that higher scores on NAS increase the risk of having acted aggressively in the past and of acting aggressively in the future.