The purpose of this study was to identify preventive effects of the Incredible Years (IY) teacher classroom management (TCM) programme, which is employed in Norwegian kindergartens for 3-year-old to 6-year-old children. IY TCM is a universal preventive intervention intended to reduce the frequency of inappropriate types of behaviour and support children’s social- and emotional competence in kindergartens and schools. IY TCM was implemented in 46 kindergartens and compared to 46 matched kindergartens that did not receive the intervention. A total of 1049 children took part in the trial, 511 from the IY TCM group and 538 from the comparison group. Children’s behaviours were measured using well-validated instruments at the beginning and end of a school-year cycle, approximately 9 months later. Findings showed promising reductions in aggression, internalising and attention problems, and increases in social competence in the IY TCM kindergartens. In a sub-sample comprising children who scored equal to or above the 90th percentile on aggressive behaviour before the intervention, promising developments were found in social competence among the children in IY TCM kindergartens. The IY TCM lead to promising preventive effects in young children’s aggressive behaviours, internalisation, attention problems, and social competence in kindergartens in Norway. However, even if the changes were statistically significant, effect sizes were small. For children with severe behaviour problems, few positive results were found.