Background: Adolescence is a critical period for physical, psychological, and social development. However, traditional school bullying and cyberbullying have emerged as major public health issues adversely affecting adolescent mental health. Aim: To estimate the prevalence of school bullying and cyberbullying among Mongolian adolescents, evaluate their impacts on psychosocial difficulties, self-harm, and suicidal ideation, and identify protective factors. Materials and Methods: A cross-sectional study was conducted under the international Global Child and Adolescent Mental Health Study (GCAMHS) framework. A randomized sample of 2936 adolescents aged 11–17 years was recruited from public schools across Ulaanbaatar, provincial, and rural areas. Psychosocial difficulties were assessed using the Strengths and Difficulties Questionnaire (SDQ). Statistical analyses included Chi-square tests, independent t-tests, and multivariable logistic regression modeling to compute adjusted odds ratios (aOR). Results: Overall, 24.2% (n=710) of adolescents reported school bullying victimization, while 15.4% experienced cyberbullying victimization. Boys experienced higher rates of traditional school victimization than girls (28.5% vs. 20.4%, p < 0.001), whereas girls were significantly more vulnerable to cyberbullying victimization (17.4% vs. 13.1%, p=0.001). Bullied students showed significantly higher total SDQ difficulty scores compared to non-victims (18.08±6.15 vs. 10.86±5.20, p < 0.001). Multivariable logistic regression analysis revealed that cyberbullying victimization was the strongest predictor for suicidal ideation (aOR=3.62, 95% CI: 2.79–4.69) and self-harm behavior (aOR=3.08, 95% CI: 2.41–3.93). School bullying victimization also increased suicidal ideation (aOR=2.79), while school safety perception served as an independent protective factor against suicidal ideation (aOR=0.64, 95% CI: 0.53–0.76). Conclusion Traditional and cyberbullying remain prevalent among Mongolian adolescents, with cyberbullying exerting a particularly strong negative impact on psychosocial distress, self-harm, and suicidal ideation. Enhancing school safety and implementing targeted psychosocial and digital safety interventions tailored to gender-specific needs are essential.