Introduction: Environmental health services (e.g., water, sanitation, hygiene, cleaning, waste management) in healthcare facilities are important to improve health outcomes and strengthen health systems, but coverage gaps remain. The World Health Organization and United Nations Children's Fund developed WASH FIT, a quality improvement tool, to help assess and improve environmental health services. Fifty-three countries have adopted it. However, there is little evidence of its effectiveness. This systematic review evaluates whether WASH FIT improves environmental health services or associated health outcomes and impacts. Methods: We conducted database searches to identify relevant studies and extracted data on study design, healthcare facility characteristics, and inputs, activities, outputs, outcomes, and impacts associated with WASH FIT. We summarized the findings using a logic model framework and narrative synthesis. Results: We included 31 studies in the review. Most inputs and activities were described qualitatively. Twenty-three studies reported quantitative outputs, primary WASH FIT indicator scores, and personnel trained on WASH FIT. Nine studies reported longitudinal data demonstrating changes in these outputs throughout WASH FIT implementation. Six studies reported quantitative outcomes measurements; the remainder described outcomes qualitatively or not at all. Common outcomes included allocated funding for environmental health services, community engagement, and government collaboration, changes in knowledge, attitudes, or practices among healthcare staff, patients, or community members, and policy changes. No studies directly measured impacts or evaluated WASH FIT against a rigorous control group. Conclusions: Available evidence is insufficient to evaluate WASH FIT's effects on outputs, outcomes, and impacts. Further effort is needed to comprehensively identify the inputs and activities required to implement WASH FIT and to draw specific links between changes in outputs, outcomes, and impacts. Short-term opportunities exist to improve evidence by more comprehensive reporting of WASH FIT assessments and exploiting data on health impacts within health management information systems. In the long term, we recommend experimental studies. This evidence is important to ensure that funding invested for WASH FIT implementation is used cost-effectively and that opportunities to adapt and refine WASH FIT are fully realized as it continues to grow in use and influence.