Systematic reviews of patient-reported outcome measures (PROMs) are important tools to select the most suitable PROM for a study or clinical application. Conducting these reviews is challenging, and the quality of these reviews needs to be improved. We updated the COSMIN guideline for systematic reviews of PROMs, including the COSMIN Risk of Bias checklist, and the COSMIN criteria for good measurement properties. Adaptations to the methodology were based on our experience with applying the COSMIN guideline, through discussions among the authors, and results from two related Delphi studies. The updated guideline places more emphasis on key aspects that are often missing or sub optimally conducted in published systematic reviews of PROMs, such as formulating a well-defined research question and developing a comprehensive search strategy, assessing risk of bias, applying criteria for good measurement properties, summarizing results, and grading the quality of the evidence. We also stress the importance of evaluating the measurement properties of each subscale of a PROM separately and evaluating content validity of all included PROMs. The quality of systematic reviews of PROMs can be improved by using this updated version of the COSMIN guideline for systematic reviews of PROMs. Improved quality will lead to better PROM selection and increased standardization of PROM use.