Previous studies showed that hostile intent attribution (HIA) was significantly correlated with and contributed to the development of aggression in children. Studies that directly examined the factors that explained the relationship between HIA and aggression are lacking. Hence, this study investigated (a) the correlation between HIA and aggression and (b) the variables (hyperactivity, prosociality, and collaborative problem-solving) that mediated the relationship between HIA and aggression in Japanese children aged 4-9 years. The participants were 180 children and their caregivers. First, the caregivers reported their children's aggression, hyperactivity, prosociality, and collaborative problem-solving through questionnaires. Next, the children worked on an HIA task. The results showed a weak positive correlation between HIA and aggression. Furthermore, significant indirect effects were observed among all the mediation models. The model that contained all three mediators yielded the smallest Akaike Information Criterion value. In this model, the indirect effect was significant only for the path with hyperactivity as the mediator. These findings provide several suggestions for revealing the mechanism of the relationship between HIA and aggression during childhood. Notably, children's hyperactivity was suggested to play a particularly important role in the relationship between HIA and aggression.