Based on 4 annual waves of data from a large sample of Chinese college students (N = 2,329, Mage = 18.40 years old, SD = .85; 63.10% females), this study examines the within-person and between-person effects in the association between problematic Internet use (PIU) and mental health issues. Results of analyses using the developmental equilibrium model (i.e., an autoregressive, cross-lagged panel model) demonstrate a reciprocal positive association between PIU and mental health issues consistently across waves. In contrast, results of analyses utilizing the random intercept, cross-lagged panel model (i.e., a model that can disaggregate within-person and between-person effects) indicate a unidirectional positive within-person effect from PIU to mental health issues (rather than the reverse) consistently over time, while controlling for the between-person effects that exist when comparing different individuals. Such findings highlight the importance of disaggregating within-person and between-person effects in understanding the nature of the temporal dynamics of the association between PIU and mental health. (PsycInfo Database Record (c) 2020 APA, all rights reserved).