Aim: There are few population-based studies on bullying behaviour among preschool children. The aims of the study were to investigate the prevalence of bullying behaviour among four-year-old children, as reported by their parents, the prevalence of types of bullying behaviour and the associations between bullying behaviour and psychosocial factors. Methods: This study was based on a population-based study sample of 931 children who attended their check-up at a child health clinic at four years of age. Parents completed the questionnaire about their child’s bullying behaviour and risk factors during the check-up. Results: Bullying behaviour, especially being both a bully and a victim, was a common phenomenon among four-year-old children. Being a bully or both a bully and victim were most strongly associated with conduct problems, while being a victim was associated with somatic symptoms and peer problems. Conclusion: Bullying behaviour was frequently found in preschool children and associated with a wide range of other problems, which indicate that routine checking of bullying behaviour should be included in child health clinic check-ups. Bullying prevention programmes are usually targeted at school-aged children, but this study highlights the importance of focusing already on preschool children.