Deficits in emotional processing are considered to underlie a range of psychopathologies, and thus constitute a crucial target to improve mental health. Given the broad consensus on the importance of emotional processing during psychotherapy to reduce symptomology, it is surprising that there is no self-report scale designed to measure this construct. The purpose of this study was to develop the Emotional Processing Self-Report (EP-SR), a practical easy-to-use tool for assessing client emotional processing during therapeutic sessions. In Study 1, 53 items were developed according to theory and evaluated by experts. Exploratory factor analyses of the newly created EP-SR items were administered in a cross-sectional design to 124 individuals in treatment. In Study 2, multilevel exploratory factor analyses were performed on a sample of 114 patients who completed the EP-SR after each therapy session (1956 measurements). The results indicated that the EP-SR could be used to assess three independent lower order emotional processes: emotional expression, regulation, and understanding emotional meaning. In Study 3, multilevel confirmatory factor analysis was conducted on additional sample of 77 patients who completed the EP-SR after each therapy session (1,423 measurements). Results confirmed the factor structure of the EP-SR. The internal consistency, convergent, incremental, and predictive validity of the EP-SR were confirmed by showing its association with clients' process and outcome measures. The results of these three studies support the reliability and validity of the EP-SR and its clinical importance. Given its user-friendliness and strong psychometric properties, the EP-SR emerges as a good instrument to assess individuals' emotional processing.