Social identity is a core aspect of procedural justice theory, which predicts that fair treatment at the hands of power holders such as police expresses, communicates and generates feelings of inclusion, status and belonging within salient social categories. In turn, a sense of shared group membership with power-holders, with police officers as powerful symbolic representatives of “law-abiding society”, engenders trust, legitimacy and cooperation. Yet, this aspect of the theory is rarely explicitly considered in empirical research. Moreover, the theory rests on the under-examined assumption that the police represent one fixed and stable superordinate group, including the often marginalised people with whom they interact, and that it is only superordinate identification that is important to legitimacy and cooperation. In this paper we present results from two UK-based studies that explore the identity dynamics of procedural justice theory. We reason that the police represent not only that the ‘law-abiding, national citizen’ superordinate group, but also a symbol of order/conflict and a range of connected social categories that can generate relational identification. First, we use a general population sample to show that relational identification with police, as well as identification as a ‘law-abiding citizen’, mediate some of association between procedural justice and legitimacy and are both stronger predictors of cooperation than legitimacy. Second, a sample of people living on the streets of London is used to explore these same relationships among a highly marginalised group for whom the police might represent a salient outgroup. We find that relational and superordinate identification are both strong positive predictors of cooperation, while legitimacy is not. These results have important implications for our understanding of both police legitimacy and public cooperation, as well as the extent to which police activity can serve to include—or exclude—members of the public.