ImportanceGender-affirming care is a key clinical area that can benefit from implementation of patient-reported outcome measures (PROMs). Identifying barriers to and enablers of PROM implementation is needed to develop an evidence-based implementation strategy.ObjectiveTo identify (1) PROMs previously implemented for gender-affirming care and constructs measured, (2) how patients completed PROMs and how results were reported and used, and (3) barriers to and enablers of PROM implementation.Evidence ReviewIn this systematic review, PubMed, Embase, MEDLINE, PsycINFO, CINAHL, and Web of Science were searched from inception to October 25, 2021, and updated on December 16, 2022. Gray literature was searched through gray literature database, online search engine, and targeted website searching. Inclusion criteria were (1) original articles of (2) a formally developed PROM or ad hoc instrument administered for gender-affirming care to (3) patients accessing gender-affirming care. The Critical Appraisal Skills Programme tool was used to evaluate quality of included studies. This review was registered on PROSPERO (CRD42021233080).FindingsIn total, 286 studies were included, representing 85 395 transgender and nonbinary patients from more than 30 countries. A total of 205 different PROMs were used in gender-affirming care. No studies described using an implementation science theory, model, or framework to support PROM deployment. Key barriers to PROM implementation included issues with evidence strength and quality of the PROM, engaging participants, and PROM complexity. Key enablers of PROM implementation included using PROMs validated for gender-affirming care, implementing PROMs able to be deployed online or in person, implementing PROMs that are shorter and reduce patient burden, engaging key stakeholders and participants as part of developing an implementation plan, and organizational climate.Conclusions and RelevanceIn this systematic review of barriers to and enablers of PROM implementation in gender-affirming care, PROM implementation was inconsistent and did not follow evidence-based approaches in implementation science. There was also a lack of patient input in creating implementation strategies, suggesting a need for patient-centered approaches to PROM implementation. Frameworks created from these results can be used to develop evidence-based PROM implementation initiatives for gender-affirming care and have potential generalizability for other clinical areas interested in implementing PROMs.