Abstract: Objectives: When it comes to interviewing suspected terrorists, global evidence points to harsh interrogation procedures, despite the likelihood of false positives. How can the state maintain an effective counterterrorism policy while simultaneously protecting civil rights? Until now, the shroud of secrecy of “national security” practices has thwarted attempts by researchers to test apparatuses that engender fair interrogation procedures. The present study aims to test one approach: the use of a “procedural justice checklist” (PJ Checklist) in interviews of suspected terrorists by counterterrorism police officers in port settings. Methods: Using a clustered randomized controlled field test in a European democracy, we measure the effect of implementing Procedural Justice (PJ) Checklists in counterterrorism police settings. With 65 teams of officers randomly-assigned into treatment and control conditions, we compare post-interrogation surveys of suspects (n = 1418) on perceptions of legitimacy; obligations to obey the law; willingness to cooperate with the police; effectiveness of counterterrorism measures; distributive justice; feelings of social resistance to the state; and PJ. A series of multi-level linear, logistic, and ordered logit regression models are used to estimate the treatment effect, with Hedges’ g and odds ratios used for effect sizes. Results: When compared with control conditions, implementing a policy of PJ Checklist causes statistically significant and large enhancement in all measured dimensions, including the willingness of suspects to obey the law (g = 1.022 [0.905, 1.138]), to cooperate with the police (g = 1.118 [0.999, 1.238]), distributive justice (g = 0.993 [0.880, 1.106]), effectiveness (g = 1.077 [0.959, 1.195]), procedural justice (g = 1.044 [0.930, 1.158]), and feelings of resistance towards the state (g = − 0.370 [− 0.259, − 0.482]). Conclusions: PJ checklists offer a simple, scalable means of improving how state agents interact with terrorism suspects. The police can use what is evidently a cost-effective tool to enhance legitimacy and cooperation with the police, even in a counterterrorism environment.