Background: The Patient-Reported Outcome Measurement Information System Upper Extremity (PROMIS UE) questionnaire has been validated as an effective and efficient outcome measure after rotator cuff repair (RCR). However, definitions of clinically significant outcomes used in interpreting this outcome measure have yet to be defined. Purpose: To define clinically significant outcomes of the PROMIS UE questionnaire in patients undergoing arthroscopic RCR. Study Design: Cohort study (diagnosis); Level of evidence, 3. Methods: We reviewed charts of consecutive patients undergoing RCR in our institution between 2017 and 2018 and included patients who were administered the PROMIS UE before surgery and 12 months after surgery. At 12 months postoperatively, patients were asked domain-specific anchor questions regarding their function and satisfaction after surgery, which were then used to determine the minimal clinically important difference (MCID), Patient Acceptable Symptomatic State (PASS), and substantial clinical benefit (SCB) using receiver operating characteristic and area under the curve (AUC) analysis. Univariate and multivariate logistic regression analysis was utilized to identify patient factors associated with clinically significant outcomes. Results: A total of 105 patients with RCR and minimum 12-month postoperative PROMIS UE were included in the analysis. The defined clinically significant outcomes were 4.87 for the MCID using a distribution-based method, 7.95 for the SCB (sensitivity, 0.708; specificity, 0.833; AUC, 0.760), and 39.00 for the PASS (sensitivity, 0.789; specificity, 0.720; AUC, 0.815). Among respondents, 79.0%, 62.9%, and 64.8% achieved the MCID, SCB, and PASS score thresholds, respectively. Workers’ compensation was negatively associated with achievement of the PASS. Lower preoperative PROMIS UE scores were associated with obtaining the MCID (odds ratio [OR], 0.871; P = .001) and the SCB (OR, 0.900; P = .040), whereas higher preoperative scores were predictive of achieving the PASS (OR, 1.111; P = .020). Conclusion: This study defines the clinically significant outcomes for the PROMIS UE after RCR, of which the majority of patients achieved the MCID, PASS, and SCB at 12 months after surgery. These thresholds should be considered in future study design and interpretation of PROMIS UE in patients with RCR.