Reviewing complex interventions is challenging because they include many elements that can interact dynamically in a nonlinear manner. A systems perspective offers a way of thinking to help understand complex issues, but its application in evidence synthesis is not established. The aim of this project was to understand how and why systems perspectives have been applied in evidence synthesis. A methodological mapping review was conducted to identify papers using a systems perspective in evidence synthesis. A search was conducted in seven bibliographic databases and three search engines. A total of 101 papers (representing 98 reviews) met the eligibility criteria. Two categories of reviews were identified: (1) reviews using a "systems lens" to frame the topic, generate hypotheses, select studies, and guide the analysis and interpretation of findings (n = 76) and (2) reviews using systems methods to develop a systems model (n = 22). Several methods (e.g., systems dynamic modeling, soft systems approach) were identified, and they were used to identify, rank and select elements, analyze interactions, develop models, and forecast needs. The main reasons for using a systems perspective were to address complexity, view the problem as a whole, and understand the interrelationships between the elements. Several challenges for capturing the true nature and complexity of a problem were raised when performing these methods. This review is a useful starting point when designing evidence synthesis of complex interventions. It identifies different opportunities for applying a systems perspective in evidence synthesis, and highlights both commonplace and less familiar methods.