Within social psychology, procedural justice theory has been used to understand variations in compliance with legal authorities such as police. Thus, it may help explain variation in cooperation and compliance in encounters between police officers and people with serious mental illness (SMI), which are often fraught with difficulty and risk. In this paper, we examine the extent to which perceptions of procedural justice among persons with SMI are associated with self-reported levels of cooperation and resistance in encounters with police. We also examine stigma and encounter type as potential moderators of the procedural justice effect. 154 persons with serious mental illness who reported a police contact within the past year were interviewed using the newly developed Police Contact Experience Survey (PCES), which includes questions about the characteristics of the contact, perceived procedural justice (PPJ) and degree of cooperation and resistance. Participants also completed the Link Perceived Devaluation and Discrimination Scale (PDS). Findings suggest that greater PPJ is associated with more cooperation and less resistance. The effect on cooperation, however, is moderated by both perceived stigma and the type of encounter. The direct effect of perceived stigma (PDS) on cooperation was unexpected, with higher perceived stigma associated with greater cooperation. Findings underline the importance of both procedurally just treatment in police interactions with vulnerable individuals and further efforts to reduce the stigma of mental illness.