The ability to regulate one’s emotions is crucial to engaging successfully in social contexts. Difficulties in emotion regulation are seen in multiple psychiatric disorders, prompting an increased interest in the concept. Suitable methods for assessing emotion regulation, however, are lacking. In this study, we investigated the interrater and intrarater reliability, construct validity, and content validity of a new observational method for evaluating children’s emotion regulation abilities (a complex puzzle task) in a sample of 62 children without psychiatric disorders and 23 children with attention-deficit/hyperactivity disorder (ADHD) aged 7–12, using intra-class correlation coefficients for the reliability analyses and Spearman’s rank-order correlations for analyses of convergent and discriminant validity. A panel of experts examined the content validity of the test, and Mann–Whitney U-tests were used to investigate the ability of the test to differentiate the non-clinical group from the ADHD group. Results showed a high level of interrater and intrarater reliability of the test. There was mixed evidence for convergent and discriminant validity as expected due to the novelty and experimental nature of the test, making it difficult to compare with questionnaire-based measures. Content validity analysis was satisfactory, and the group comparison showed that the test differentiated the groups on the primary outcome measure. Overall, the measure demonstrated high feasibility and satisfactory psychometric properties. The generic nature of the test makes it suitable for use across psychiatric disorders and age groups with potential relevance in both research and clinical settings.