Background: This study aimed to assess the relationship between leisure time spent watching television (TV) and at a computer and aggressive and violent behaviour in children and adolescents. Methods: In this nationwide study, 14,880 school students, aged 6-18 years, were selected by cluster and stratified multi-stage sampling method from 30 provinces in Iran. The World Health Organization Global School-based Health Survey questionnaire (WHO-GSHS) was used. Results: Overall, 13,486 children and adolescents (50.8% boys, 75.6% urban residents) completed the study (participation rate 90.6%). The risk of physical fighting and quarrels increased by 29% (OR 1.29, 95% CI 1.19-1.40) with watching TV for >2 hr/day, by 38% (OR 1.38, 95% CI 1.21-1.57) with leisure time computer work of >2 hr/day, and by 42% (OR 1.42, 95% CI 1.28-1.58) with the total screen time of >2 hr/day. Watching TV or leisure time spent on a computer or total screen time of >2 hr/day increased the risk of bullying by 30% (OR 1.30, 95% CI 1.18-1.43), 57% (1.57, 95% CI 1.34-1.85) and 62% (OR 1.62, 95% CI 1.43-1.83). Spending >2 hr/day watching TV and total screen time increased the risk of being bullied by 12% (OR 1.12, 95% CI 1.02-1.22) and 15% (OR 1.15, 95% CI 1.02-1.28), respectively. This relationship was not statistically significant for leisure time spent on a computer (OR 1.10, 95% CI 0.9-1.27). Conclusions: Prolonged leisure time spent on screen activities is associated with violent and aggressive behaviour in children and adolescents. In addition to the duration of screen time, the association is likely to be explained also by the media content.