Participatory systems mapping methods are increasingly used to explore complex population health challenges by involving diverse stakeholders in co-producing causal diagrams. However, the field lacks tools that support thoughtful design and selection of methods. This paper introduces the Participatory Systems Mapping Design Framework, developed within a wider methodological research programme through evidence synthesis, case study development and international expert consultation. The Framework is intended to support researchers, practitioners and policymakers with varied systems mapping experience and is structured around three core considerations: (1) What is the scope of the project? (2) What is the added value of a participatory approach? and (3) Which factors may affect capacity to use a participatory systems mapping method? Each is supported by scaffolded questions and an open-access tool, alongside a comprehensive guidance document, to enable reflective, context-sensitive design and use. As a deliberative design resource rather than a prescriptive decision aid, the Framework encourages broader use of underused methods and supports greater transparency, inclusivity and methodological rigour. It also aims to strengthen the design and reporting of participatory processes and highlights the importance of planning for the use and value of system maps beyond the lifespan of projects. By addressing gaps in design rationale and stakeholder involvement, the Framework aims to enhance the relevance, coherence and impact of participatory systems mapping in population health. This deliberative tool provides a foundation for continued methodological development, and we invite testing and refinement to strengthen its contribution in understanding and generating solutions for complex population health problems.