The article documents how the city of Seoul under Mayor Park Won-soon has become a leader in fostering social innovation. The Seoul Municipal government has achieved huge success in developing a policy ecosystem that encourages social innovation. That is because Seoul—the country’s capital and home to 20 percent of South Korea’s population—actively welcomes input from residents and has been experimenting with ways to encourage citizen engagement, and to ensure that when citizens raise an issue, the city is poised to work efficiently and collaboratively towards a solution. One of the key actors within the local government is the Seoul Innovation Planning office, which serves as a key coordinator of all social innovation related policy work, acting as the bridge across different governmental bodies, and serving as the financier of projects. This case study highlights the importance of an influential policy champion, the benefit of clarifiying roles within the local government, and the need for policymakers to create an enabling environment to foster social innovation at scale.