Purpose: Social media use (SMU) has become an intrinsic part of adolescent life. Negative consequences of SMU for adolescent health could include exposures to online forms of aggression. We explored age, gender, and cross-national differences in adolescents' engagement in SMU, then relationships between SMU and victimization and the perpetration of cyber-bullying. Methods: We used data on young people aged 11e15 years (weighted n ¼ 180,919 in 42 countries) who participated in the 2017e2018 Health Behaviour in School-aged Children study to describe engagement in the three types of SMU (intense, problematic, and talking with strangers online) by age and gender and then in the perpetration and victimization of cyber-bullying. Relationships between SMU and cyber-bullying outcomes were estimated using Poisson regression (weighted n ¼ 166,647 from 42 countries). Results: Variations in SMU and cyber-bullying follow developmental and gender-based patterns across countries. In pooled analyses, engagement in SMU related to cyber-bullying victimization (adjusted relative risks ¼ 1.14 [95% confidence interval (CI): 1.10e1.19] to 1.48 [95% CI: 1.42e1.55]) and perpetration (adjusted relative risk ¼ 1.31 [95% CI: 1.26e1.36] to 1.84 [95% CI: 1.74e1.95]). These associations were stronger for cyber-perpetration versus cyber-victimization and for girls versus boys. Problematic SMU was most strongly and consistently associated with cyber-bullying, both for victimization and perpetration. Stratified analyses showed that SMU related to cybervictimization in 19%e45% of countries and to cyber-perpetration in 38%e86% of countries. Conclusions: Accessibility to social media and its pervasive use has led to new opportunities for online aggression. The time adolescents spend on social media, engage in problematic use, and talk to strangers online each relate to cyber-bullying and merit public health intervention. Problematic use of social media poses the strongest and most consistent risk.