IntroductionConsiderable evidence supports an association between poor impulse control (impulsivity) and violent crime. Furthermore, impulsivity and aggression has been associated with reduced levels of serotonergic activity in the brain. Selective serotonin reuptake inhibitors (SSRIs) are a class of anti­depressants that aim to regulate brain serotonin concentrations. Several small studies in psychiatric populations have administered SSRIs to impulsive­–aggressive individuals, resulting in reduced impulsivity, anger, aggression and depression. However, no clinical trial has been undertaken in a criminal justice population. This protocol describes the design and implementation of the first systematic study of the potential benefits of SSRIs in impulsive­­–violent offenders who are at high risk of reoffending.Methods and analysisA randomised, double-blinded, multicentre trial to test the clinical efficacy of an SSRI, sertraline hydrochloride, compared with placebo on recidivism and behavioural measures (including impulsivity, anger, aggression, depression and self-reported offending) over 12 months. 460 participants with histories of violence and screening positive for impulsivity are recruited at several local courts and correctional service offices in New South Wales, Australia.Ethics and disseminationResults will be submitted for publication in a peer-reviewed journal. Possible implications of the effectiveness of this pharmacological intervention include economic benefits of reducing prison costs and societal benefits of improving safety. This study has received ethical approval from the University of New South Wales, Aboriginal Health & Medical Research Council, Corrective Services NSW and the NSW Justice Health and Forensic Mental Health Network.Trial registration numberACTRN12613000442707.