Sleep problems affect the majority of patients with mental disorders and are very clinically relevant. However, they are often not assessed and treated appropriately and, to date, there is no established screening routine for sleep disturbances in this specific patient group. For patients with Parkinson disease, the Scales for Outcomes in Parkinson’s Disease–Sleep (SCOPA-Sleep) questionnaire is widely used. This questionnaire has excellent psychometric properties. It consists of 2 scales for assessing nighttime sleep problems (NS) and daytime sleepiness (DS). As the items in the scales are not disease-specific, the goal of this study was to validate the SCOPA-Sleep for use in patients with mental disorders. A total of 370 German patients with mental disorders completed the SCOPA-Sleep as well as a multitude of other instruments for assessment of validity [eg, Pittsburgh Sleep Quality Index (PSQI), Epworth Sleepiness Scale (ESS)]. For validity estimation, 627 healthy subjects were also included in this study. Internal consistency was good with coefficients of 0.806 and 0.772 for the SCOPA-NS and the SCOPA-DS. Factor analysis revealed 2 factors which could each be assigned to one of the subscales. Convergent validity was high with correlations of 0.672 between the SCOPA-NS and the PSQI and 0.644 between the SCOPA-DS and the ESS. The German version of the SCOPA-Sleep showed good diagnostic accuracy, resulting in an area under the receiver operating curve of 0.959 for the SCOPA-NS and 0.728 for the SCOPA-DS. For patients with mental disorders, the SCOPA-Sleep proved to be an economical, reliable, and valid instrument for assessing NS and DS which is well accepted by patients. It can be recommended for routine assessment of both NS and DS.