This article discusses the idea of city as a platform. The analysis focuses on the forms and implications of citizen involvement in publicly-supported participatory innovation platforms that facilitate urban economic development in the welfare society context. The discussion opens with a review of the smart city discourse, which in the context of economic development policy translates into cities’ need to support innovativeness by creating smart environments. Participatory innovation platform is a prime example of such an environment. The empirical section discusses three cases, those of the Finnish cities of Helsinki, Tampere, and Oulu. The analysis shows that platformization in the first half of the 2010s became a strategic focal area supported by national and EU programs. Platforms are used to support both urban revitalization and economic development, of which the former is based on representative and the latter on instrumental modes of participation. Platforms are well integrated with city governments, even though they vary greatly in terms of organizational forms and scopes. Democratic culture, welfarism, and redistributive policy provide contextual support for platformization by strengthening social inclusion, taming the growth machine, and easing the tensions between pro-growth and anti-growth coalitions.