In cities across the world, local food networks aim to make food systems more sustainable and secure for all. As part of that effort, some of these networks also seek to introduce social innovation in the mode of selling food, namely as a way to initiate a broader transition of the sector. Based on two years of action research conducted together with promoters of Montreal’s seasonal markets, this article offers an account of the co-constructed narrative of a transition of the agri-food sector. On the one hand, transition theory anticipates that the transition to sustainability of the agri-food sector would depend on the protection and empowerment of innovative ‘niches’ that are facing the locked-in structure of the agri-food ‘sociotechnical regime’. Yet, on the other hand, the seasonal markets do not fit well in this portrait: they are shown to evolve at the intersection of the sociotechnical regime and innovative niches. For this reason, they are subject to regime rules and become difficult to protect as an entity. As such, seasonal markets face ‘structuring tensions’ that generate both practical dilemmas and innovative solutions in their modes of organization. These solutions, however, rely on webs of resources and supports that constitute ‘key relations’ for unlocking the agri-food regime rules. It is through managing these tensions and relations that the seasonal markets end up reconfiguring social and material relations and providing solutions for food security and a more sustainable food system. Therefore, we argue that the structuring tension and key relation concepts are useful for understanding the dynamics of social innovation in the transition to sustainability in food systems.