This thesis is a study of legitimacy and protest policing in contemporary Hong Kong through a framework called the legitimist game. Legitimacy describes power relations that are recognised as rightful. It has become an established concept in criminology, with a growing body of literature within the last two decades. However, the literature has focused almost entirely on quantitative analyses of community surveys of public perceptions of police legitimacy and the implications for public behaviour. Qualitative studies of legitimacy are rare; more crucially, legitimacy at protests remains largely unexplored. My research seeks to fill this gap in our knowledge by exploring the experiences and expectations of protestors as well as their perceptions of police. The data come from the 2016-17 and 2019 research phases, 17 months of field observations and a total of 40 in-depth interviews. Insights from Wittgenstein’s language game (1958) are applied, for the first time, to a criminological analysis of legitimacy. The bodies of literature on social movement and protest policing are reviewed. It is found possible to improve the dialogic model of legitimacy (Tankebe 2013; Bottoms and Tankebe 2012) by using a game metaphor. The findings are presented in terms of 1) games in the field, and 2) experiences and perceptions; these are drawn from the field and interview data respectively. The field data from the two phases are separately discussed to inform us of the situations in these periods. Major roles are observed. Cooperative moves are seen to become aggressive due to both ideology and sentiments. The moves are made according to rules, which are turning from being ritualized to sentimental. The protestors’ experience resonates with Brodeur’s (1983) idea of “high policing” with nationalist elements. There are issues and misunderstandings around their basic expectations. They also gather self-legitimacy through their definition of Hong Kong, charisma, and an intuitive sense of legitimacy. The thesis ends with summaries with respect to research questions and chapters, as well as discussions of limitations, implications and the matter of change.