BACKGROUND: Physical activity can be understood in terms of socioecological determinants, which are prerequisites to successfully promote physical activity. Communities play an important role in this context, as they can create health-protective conditions. However, the design, implementation, and evaluation of socioecological approaches often do not follow a systematic and theory-driven approach. Changes in environmental conditions are determined by the behavior and decisions of so-called change agents or local decision-makers, such as mayors. Therefore, it is important to identify the determinants that influence decision-making processes to be able to change the decisional behavior of change agents to promote physical activity in their communities. In addition, there is a need for multipliers (e.g., employees in health offices) to be qualified to systematically implement socioecological-based interventions in communities. OBJECTIVES AND PROJECT PROCESS: The research project "Systematic planning and implementation of decision-making and implementation processes of socioecological-based physical activity promotion in the community to reduce social inequity" (with the German acronym EUBeKo) was funded within the framework of the funding scheme "Physical Activity and Physical Activity Promotion" of the German Federal Ministry of Health. It takes both the local change agents and multipliers into account and is inspired by intervention mapping. This article describes the project EUBeKo with its two research questions on the role and competences of multipliers, and on the factors influencing local decision-making processes as well as the planning and implementation of socioecological-based physical activity promotion in two model communities, and the dissemination strategies (e.g., project website). DISCUSSION AND CONCLUSIONS: The strengths of the project include the systematic and theory-based preparation and implementation of a process of socioecological-based physical activity promotion in the community with a special focus on planning and decision-making processes as well as on the target groups of local multipliers and decision-makers. Challenges are found in the theory-practice transfer and the impact of the coronavirus disease 2019 (COVID-19) pandemic on project implementation.