This study aimed to explore the potential mediating role of sleep quality in the effect of physical activity (PA) intervention for improving executive functions (EFs) in children with attention-deficit hyperactivity disorder (ADHD). Participants aged 6 to 12 years old with a formal ADHD diagnosis were recruited from a local hospital. A total of 80 eligible children with ADHD were randomized to an intervention group for 12 consecutive weeks of PA training (three times per week, 60 min per session) (n = 40; Mage = 8.37, 75% boys) or a wait-list control group (n = 40; Mage = 8.29, 80% boys). Three core EFs (inhibitory control, working memory, and cognitive flexibility) were assessed by neurocognitive tasks, and sleep quality was measured by the Chinese version of the Pittsburgh Sleep Quality Index. The bootstrapping method was used to test PA intervention effects on EFs and on potential variables of sleep quality after intervention and to test whether there were indirect effects of the intervention on EFs via mediators of sleep. The results showed that the PA intervention had a direct effect on sleep latency reduction (β = - 0.26, 95%CI - 0.47 to - 0.06) and cognitive flexibility improvement (decrease in completion time) (β = - 0.30, 95%CI - 0.50 to - 0.09). Furthermore, change in sleep latency significantly mediated the effects of PA intervention on cognitive flexibility (β = - 0.084, 95%CI - 0.252 to - 0.001). The findings suggest that sleep latency could be a crucial behavioral mediator of PA intervention in improving cognitive flexibility among children with ADHD.