Background: The Utrecht Symptom Diary (USD) is a Dutch and adapted version of the Edmonton Symptom Assessment System, a patient-reported outcome measurement (PROM) tool to asses and monitor symptoms in cancer patients. This study analyses the validity and responsiveness of the USD and the cutoff points to determine the clinical significance of a symptom score. Methods: Observational longitudinal cohort study including adult in- and outpatients treated in an academic hospital in the Netherlands who completed at least one USD as part of routine care (2012-2019). The distress thermometer and problem checklist (DT&PC) was used as a reference PROM. Content, construct and criterion validity, responsiveness, and cutoff points are shown with prevalences, area under receiver operating characteristic (ROC) curve, Chi-squared test, Wilcoxon signed-rank test, and positive and negative predictive values, respectively. Results: A total of 3913 patients completed 22 400 USDs. Content validity was confirmed for all added USD items with prevalences of ≥22%. All USD items also present on the DT&PC demonstrated a good criterion validity (ROC >0.8). Construct validity was confirmed for the USD as a whole and for the items dry mouth, dysphagia and well-being (P <.0001). USD scores differed significantly for patients when improving or deteriorating on the DT&PC which confirmed responsiveness. Optimal cutoff points (3 or 4) differed per symptom. Conclusion: The USD is a valid 12-item PROM for the most prevalent symptoms in cancer patients, which has content, criterion, and construct validity, and detects clinically important changes over time, in both curative and palliative phase.