Background and aims: The Spectrum Hypothesis posits that various forms of problematic usage of the internet (PUI) constitute distinct yet related constructs. However, empirical validation of this hypothesis has largely relied on cross-sectional data, leaving gaps in understanding the co-developmental trajectories of these behaviors during adolescence, their microsystem predictors, and how identified trajectories are associated with mental health outcomes. This study thus aimed to: (a) identify the heterogeneous co-developmental trajectories of problematic social media use, short video use, and internet game use; (b) examine the microsystem-level factors that predict membership in these distinct trajectories; and (c) investigate how these trajectory classes are associated with mental health outcomes. Methods: A total of 1,975 Chinese middle school students (M _age = 13.51, 52.56% girls) completed measures on three occasions across one year.Results: Parallel process latent class growth modeling revealed five distinct trajectory groups: Low-Stable (61.0%), High-Increasing (8.1%), High-Stable Gaming and Moderate-Decreasing Social Media (15.5%), High-Stable Social Media and Short Video (6.9%), and Moderate-Increasing Social Media (8.4%). Harsh parenting, teacher-student conflict, and bullying victimization predicted worsening co-developmental trajectories. In addition, the High-Increasing class had higher risks for adverse mental health outcomes (i.e., depression, anxiety, and suicidality) compared to the Low-Stable class, whereas other high-risk classes also showed poorer outcomes (though less severe). Conclusions: These findings support the Spectrum Hypothesis from a developmental perspective, highlighting the importance of considering heterogeneity in understanding the co-developmental patterns of PUI forms, their microsystem predictors, and cumulative effects on adolescent mental health.