Research on new product development has emphasized the importance of integrating customers, employees, and suppliers in an organization’s innovation processes. While several studies evaluate respective participatory processes, there is, surprisingly, no consensus on which dimension participation quality consists of, and how to measure them consistently. The present study contributes to the literature by identifying the dimensions of participation quality and by constructing a participation quality scale that includes six dimensions, namely (1) project-related resources, (2) early involvement, (3) degree of influence, (4) transparency of processes, (5) incentive mechanisms, and (6) voluntariness of participation. Furthermore, a 24-item measure of participation quality is developed using a mixed-method design. Results of our study show that the developed measure impacts important innovation-related outcomes such as innovation performance, acceptance of the innovation, and intention to participate in future innovation projects. Given that the six identified dimensions of participation quality differentially affect these outcomes, the scale provides the opportunity to better design participatory innovation projects, and thus helps managers to integrate stakeholders more successfully in these projects.