Background and Objectives: Internet gaming disorder (IGD) is a growing public health concern, especially among adolescents. However, most IGD studies are cross-sectional, leaving its developmental course poorly understood. This study examined IGD trajectories, their predictors, and their long-term mental health outcomes among Chinese adolescents. Methods: A cohort of 16,833 adolescent gamers (mean age 5 13.40, 46.56% girls) was recruited from 76 middle schools in Zigong City through cluster sampling. Assessments were conducted in November 2020 (T1), November 2021 (T2), and November 2022 (T3). IGD was measured using the Internet Gaming Disorder Scale-Short Form (IGDS9-SF). Demographic, gaming, environmental, and psychological factors (e.g., depression, anxiety, sleep problems, suicidal ideation, conduct problems, hyperactivity, resilience, mental wellbeing, prosocial behavior) were also assessed. Growth mixture modeling (GMM) identified IGD trajectories, and logistic regression evaluated predictors and mental health outcomes at T3. Results: GMM identified four IGD trajectories: High-Risk Decreasing (4.5%), Moderate-Risk Stable (19.5%), Moderate-Risk Increasing (3.0%), and Low-Risk Stable (73.0%). Age, gaming time, anxiety, sleep problems, conduct problems, hyperactivity/inattention, and low prosocial behavior predicted worsening IGD symptoms. At T3, the Moderate-Risk Increasing group had significantly higher risks for adverse mental health outcomes compared to the Low-Risk Stable group, whereas the High-Risk Decreasing group showed no significant or minimal differences in most domains from the Low-Risk Stable group after adjusting for baseline characteristics. Discussion and Conclusions: These findings underscore the heterogeneous nature of IGD development and highlight the need for long-term monitoring and tailored interventions to mitigate adverse mental health outcomes in at-risk adolescents.  Kulcsszavak: internet gaming disorder, trajectory, adolescents, mental health outcomes