Background: Clinicians usually conduct diagnostic assessments in order to establish a diagnosis or to evaluate the effect of treatment. Two meta-analyses suggest that diagnostic assessment administered in collaboration with the patient and personalized feedback might have a therapeutic effect. Methods: We aim to conduct a systematic review and meta-analysis of the effect on symptomatology when using assessment as a therapeutic intervention for patients with psychiatric illnesses. We will search in five relevant electronic databases. Two reviewers will independently select papers following pre-defined eligibility criteria, extract data, and assess the quality of included studies. Randomized controlled trials comparing the effect of psychological assessment with other psychotherapeutic interventions in populations of patients will be included in the meta-analysis. We will extract data on symptom-related outcomes, quality of life, dropout, and re-diagnosis and use meta-analysis techniques to compute the effect size of interventions using assessment as a psychotherapeutic intervention. The review will be conducted and reported according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) statement. Risk of bias will be assessed by using the Risk of Bias tool RoB 2.0 of the Cochrane Collaboration, and the certainty of the body of evidence will be assessed with the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach. Systematic review registration: PROSPERO CRD42021270567