Social innovation brings about sustainability which is regarded as a new paradigm for development. In order to bring about social innovation, cross-sector collaboration among different actors is required. However, it is known that establishing cross-sector collaboration is very complex, especially in a penta helix model where the public administration, business, academia, third sector and citizens are all involved. The research aims to investigate the initial conditions for establishing penta helix collaboration in the context of co-produced social innovation from the perspectives of core co-producers. Through a case study of ReTuren, a co-produced public platform for waste handling and prevention in Malmö, Sweden, the research finds out four themes of initial conditions, viz. environment, resources, relationships, and strategy. It is also discussed that the significance of these conditions to the collaboration establishment can depend on the development stage of the social innovation initiative. The research also provides new insights about the unclear boundary and the flexible role of each sector in the penta helix model. Based on the findings, an adapted model of initial conditions from Bryson et al. (2015) for penta helix collaboration in social innovation is created.