The purpose of this study was to examine the differential mediational roles of perceived peer relationship stress (PPRS) in accounting for the association between cyberbullying (CB) and cybervictimization (CV) and mental health among early adolescents in cross-sectional data and longitudinal data, respectively. A total of 606 early adolescents completed questionnaires as part of a 3-year longitudinal study on three occasions at 1-year intervals. Structural equation modeling revealed that (1) compared to CB, CV showed a stronger relationship with mental health. (2) In contrast to its role in CV, PPRS did not mediate the link between mental health and CB in both cross-sectional and longitudinal models. PPRS mediated the relationship between CV and mental health in both cross-sectional and longitudinal models. Limitations and practical applications of the study were discussed.