Screens have become increasingly prevalent in children's lives. This study examined the longitudinal association between screen time and aggressive behaviour by investigating the mediating and moderating roles of language ability and parenting stress, respectively. The analysed data consisted of 1,319 children, aged 5 to 15, from the Fragile Families and Child Wellbeing Study. Results found that children's language ability (age 9) significantly mediated the relationship between screen time (age 5) and aggressive behaviour (age 15), and this mediating pathway was significantly moderated by parenting stress (age 5). Furthermore, the moderated mediation pathway was only significant among groups with medium or high parenting stress. Thus, the negative effects of screen time on aggressive behaviour through poor language ability amplified as the level of parenting stress increased.