The temporary use of vacant spaces has become integrated into formal urban development practices. This paper seeks to explore the catalytic power of temporary urbanism for sustainable urban development by focusing on the tension between temporary and permanent and the dynamics through which temporary urbanism becomes catalytic. The case study focuses on a six-year experimental period of the Hiedanranta brownfield until the city started to build the area for a new residential district in Tampere, Finland. Rich details of temporary urbanism were achieved through interviews with 27 city developers and experimental participants. The analysis revealed three tracks of catalytic mechanisms of temporary urbanism emerging from the persistent tension between temporary and permanent. By demonstrating the origins of catalytic power, this research contributes to academic and practical interests to find productive relationships between temporary urbanism, planning and sustainability governance in cities.