Healthcare professionals may employ therapeutic games as playful activities to promote rehabilitation for their patients. However, despite the importance, currently the literature does not provide multidisciplinary approaches to support designing these games. To overcome this scenario, we propose a semiotic-based approach to aid stakeholders with different backgrounds on designing therapeutic digital games. The approach results from a study of the literature and reports of the experience of a multidisciplinary team on the creation of therapeutic games. It explores Participatory Design and Organizational Semiotics methods and artifacts to promote the Problem Clarification, Interaction Modeling, Design Materialization, and Evaluation as fundamental design practices of effective solutions. The approach was evaluated to design new therapeutic games and the results suggest that it helps the team on identifying therapeutic objectives and reaching them.