Regional design and spatial planning have a long history, but their relationship and impact on planning decisions are not well understood. This study aimed to explore how regional design influences planning performances. Through case studies, it was found that regional design can improve spatial quality, enhance planning innovation, clarify political agendas, and increase planning efficiency. However, the outcomes of regional design are often unpredictable due to the complex interplay between design and planning. The study provides insights into the factors that influence the effectiveness of regional design in planning processes.