Abstract Introduction Persons in socioeconomically disadvantaged situations (PSEDS) are more susceptible and disproportionally exposed to obesogenic environments, resulting in poorer dietary outcomes and lower levels of physical activity. This study aimed to map the complex dynamic system within the community obesogenic environment driving unhealthy eating and insufficient physical activity among PSEDS in Flanders, and to identify potential local actions. Methods Participatory group model building (GMB), a qualitative system dynamics method, was used to engage PSEDS (n = 7), local decision makers (n = 7) and local stakeholders (n = 5) in two Flemish municipalities. A two-day GMB session was held in each municipality during which a causal loop diagram (CLD) was created and potential local actions were identified by considering causal pathways and reinforcing loops in the system. Results The CLD was designed around two main variables (i) healthy eating and (ii) physical activity among PSEDS. Seven sub-systems could be identified: 1) social environment, 2) facilities and activities, 3) mobility options, 4) information, 5) affordability, 6) safety (specifically for physical activity), and 7) industry and advertisement (specifically for eating behaviour). Several actions to improve the local environment to promote eating behaviour and physical activity among PSEDS were identified. Conclusions The CLD illustrates the complex interactions between individual and environmental determinants contributing to inequalities in eating behaviour and physical activity by including multiple perspectives of community members in socioeconomically disadvantaged situations, local decision makers and local stakeholders. Both the participatory GMB approach and the final CLD visualization can provide the basis for planning local actions for promoting healthy eating behaviour and physical activity among PSEDS. Key messages • Group model building is an effective approach to explore and understand a dynamic complex system through a shared understanding among diverse stakeholders. • A systems approach is valuable to provide a more comprehensive and context-specific understanding of the perceived factors contributing to inequities in the obesogenic environment.