Introduction: Describing the content and underlying theoretical principles of an intervention designed to change behavior is a fundamental step toward replicating the successful elements of interventions that have proven effective. The objective of this article is to describe the evidence-based development of a multimodal communication program, implemented by the National Institute for Prevention and Health Education, aimed at promoting physical activity among the general population and, more specifically, walking among physically inactive women. Methods: The first five stages of developing this ongoing intervention are described. These include defining the intended objectives, identifying the target populations, the behavioral change theories used, identifying effective behavioral change techniques, and the methods for implementing the program. Results: The program primarily aims to promote walking among physically inactive women through a media campaign and a smartphone app. It also addresses the general population through the development of a dedicated section on physical activity on the mangerbouger.fr website. It is based on the COM-B system—a framework for understanding behavior, the theory of planned behavior, and behavior change techniques identified as effective. Discussion: This article demonstrates the feasibility of developing a physical activity promotion intervention based on a multimodal communication strategy through the use of theory, evidence, and expertise in designing communication strategies. An evaluation of this strategy and its various components constitutes the next step in this work.