BACKGROUND: Research increasingly demonstrates a positive association between Internet Gaming Disorder (IGD) and conduct problems among adolescents. However, longitudinal data are limited, and the mediating mechanisms remain unclear. This study aimed to examine the predictive effect of IGD on conduct problems and explore the mediating role of impaired sleep quality. METHOD: A cohort of 20,137 Chinese seventh- and tenth-grade students was recruited and assessed at three time points: November 2020 (T1), 2021 (T2), and 2022 (T3). IGD, conduct problems, and impaired sleep quality were measured using the Internet Gaming Disorder Scale Short Form, the Strengths and Difficulties Questionnaire, and the Pittsburgh Sleep Quality Index, respectively. Mediation analyses were conducted to evaluate the direct and indirect effect of IGD on conduct problems, with subgroup analyses based on sex and developmental stage. RESULTS: IGD was found to be an independent risk factor for conduct problems both cross-sectionally and longitudinally. Impaired sleep quality partially mediated the relationship between IGD and conduct problems, accounting for approximately 17.3% of the total effect. Subgroup analyses revealed that the mediation effect of impaired sleep quality was more pronounced in early adolescents and varied by sex, with a stronger total and direct effect in boys. CONCLUSIONS: These findings highlight the need for comprehensive interventions targeting both IGD and impaired sleep quality, tailored to specific sexes and developmental stages, to effectively reduce conduct problems.