INTRODUCTION: The purpose of this study is to evaluate the utility of the modified frailty index-5 (mFI-5) as a predictor for postoperative complications in patients undergoing arthroscopic rotator cuff repair (RCR). METHODS: The National Surgical Quality Improvement Program database was queried for patients undergoing arthroscopic RCR between 2006 and 2016. The mFI-5, a 5-factor score comprising comorbid diabetes, hypertension, congestive heart failure, chronic obstructive pulmonary disease, and functional status limiting independence, was calculated for each patient. Multivariate logistic regression models were used to evaluate the mFI-5 score as a predictor for complications including medical complications, surgical-site infections, hospital admission, discharge to a facility, and mortality. RESULTS: In total, 24,477 patients met criteria for inclusion. The mFI-5 was a strong predictor for medical complications (P < .001), hospital admission (P < .001), length of stay (P = .007), and discharge to a facility (P = .001) but not surgical-site infections (P = .153). For each point increase in mFI-5 score, the risk for a medical complication increased by 66%, readmission by 52%, and adverse discharge by 45%. However, of all the measured complications, the mFI-5 was the strongest predictor for mortality, with the risk more than doubling for each increase in mFI-5 point (odds ratio 2.66, P = .025). CONCLUSIONS: The mFI-5 is a sensitive tool for predicting life-threatening medical complications, hospital admission, increased length of stay, adverse discharge, and mortality following arthroscopic RCR. The 5 comorbidities comprising the mFI-5 are easily obtained through the patient history, making it a practical clinical tool for identifying high-risk patients, informing preoperative counseling, and improving value-based health care. LEVEL OF EVIDENCE: Level III, prognostic.