South Korea started building smart cities in the early 2000s, much earlier than other countries. However, the smart city winter, analogous to the artificial intelligence winter, arrived in 2010 and remained for many years. There were many reasons for its arrival, but primarily it set in because smart cities were recognised as ‘products’, which usually stop developing once they are produced. This led Korea’s smart city project, u-City, to rely on one-off and fragmented development and fail to meet the high expectations of the South Korean people. Adapting its approach based on the lessons learned from the smart city winter, the South Korean government designed a new smart city strategy in 2018 with ‘city-as-a-platform’ as its core component. The city-as-a-platform strategy could reduce the cost of urban innovation, enabling continuous innovations and democratising technology. This article explains why smart cities need a city-as-a-platform strategy based on cases in South Korea.