Background Violence is a leading global public health problem and interventions in early childhood are important in the primary prevention of violence. We tested whether the Irie Classroom Toolbox, a violence-prevention, teacher-training programme reduced violence against children (VAC) by teachers and reduced class-wide child aggression. Methods In this cluster-randomised controlled trial, seventy-six preschools were randomly selected from 120 eligible preschools in Kingston, Jamaica, and were assigned (1:1) to receive the Irie Classroom Toolbox or to a control group. All teachers/classrooms in the selected schools participated in the study (n=229). The intervention was implemented from August 2015 to April 2016 and involved training teachers in classroom behaviour management. All assessors were masked to group assignment. Teacher and classroom measures were conducted in the summer school term (May/June) at baseline (2015), post-intervention (2016), and one-year follow-up (2017). The primary outcomes were observations of VAC by teachers, and class-wide child aggression at post-intervention and one-year follow-up. Trial Registration: ISRCTN11968472FindingsAll schools were included at post-intervention and follow-up. 108/119 (90·8%) intervention and 92/110 (83·6%) control teachers were assessed at post-intervention and 105/119 (88·2%) intervention and 85/110 (77·2%) control teachers at one-year follow-up. Teachers in intervention schools used significantly less VAC at post-intervention (-67·12%, 95%CI -80·71%, -53·52%, p<0.0001) and one-year follow-up (-53·86, 95%CI -71·08%, -36·65%, p<0.0001). No benefits were found for class-wide child aggression at post-intervention (effect size 0·07, 95%CI -0·16, 0·29, p=0·72) or one-year follow-up (-0·14, 95%CI -0·42, 0·16, p=0·72). Interpretation The Irie Classroom Toolbox effectively reduced VAC by Jamaican early childhood teachers. The Toolbox was designed for use with undertrained teachers working in low-resource settings and should be suitable with early childhood practitioners in other LMICs. Additional research is needed to further develop the Toolbox to reduce class-wide child aggression.