Introduction: The International Index of Erectile Function (IIEF) is a patient-reported outcome measure to evaluate erectile dysfunction and other sexual problems in men. Aim: To perform a systematic review of the measurement properties of the 15-item patient-reported outcome measure (IIEF-15) and the shortened 5-item version (IIEF-5). Methods: A systematic search of scientific literature up to April 2018 was performed. Data were extracted and analyzed according to COnsensus-based Standards for the selection of health Measurement INstruments (COSMIN) guidelines for structural validity, internal consistency, reliability, measurement error, hypothesis testing for construct validity, and responsiveness. Evidence of measurement properties was categorized into sufficient, insufficient, inconsistent, or indeterminate, and quality of evidence as very high, high, moderate, or low. Results: 40 studies were included. The evidence for criterion validity (of the Erectile Function subscale), and responsiveness of the IIEF-15 was sufficient (high quality), but inconsistent (moderate quality) for structural validity, internal consistency, construct validity, and test-retest reliability. Evidence for structural validity, test-retest reliability, construct validity, and criterion validity of the IIEF-5 was sufficient (moderate quality) but indeterminate for internal consistency, measurement error, and responsiveness. Clinical Implications: Lack of evidence for and evidence not supporting some of the measurement properties of the IIEF-15 and IIEF-5 shows the importance of further research on the validity of these questionnaires in clinical research and clinical practice. Strengths & Limitations: A strength of the current review is the use of predefined guidelines (COSMIN). A limitation of this review is the use of a precise rather than a sensitive search filter regarding measurement properties to identify studies to be included. Conclusion: The IIEF requires more research on structural validity (IIEF-15), internal consistency (IIEF-15 and IIEF-5), construct validity (IIEF-15), measurement error (IIEF-15 and IIEF-5), and responsiveness (IIEF-5). The most pressing matter for future research is determining the unidimensionality of the IIEF-5 and the exact factor structure of the IIEF-15. Neijenhuijs KI, Holtmaat K, Aaronson NK, et al. The International Index of Erectile Function (IIEF)—A Systematic Review of Measurement Properties. J Sex Med 2019;16:1078–1091.