There is increasing international interest in the use of police drug diversion schemes that offer people suspected of minor drug-related offences an educative or therapeutic intervention as an alternative to criminalisation. While there have been randomised trials of some such schemes for their effects on reducing offending, with generally positive results, less is known about the health outcomes, and what works, for whom, in what circumstances and why. This protocol reports on a realist evaluation of police drug diversion in England that has been coproduced by a team of academic, policing, health, and service user partners. The overall study design combines a qualitative assessment of the implementation, contexts, mechanisms, moderators and outcomes of schemes in Durham, Thames Valley and the West Midlands with a quantitative, quasi-experimental analysis of administrative data on the effects of being exposed to the presence of police drug diversion on reoffending and health outcomes. These will be supplemented with analysis of the cost-consequences of the evaluated schemes, an analysis of the equity of their implementation and effects, and a realist synthesis of the various findings from these different methods.