Purpose: Although previous research has established a positive association between national income inequality and socioeconomic inequalities in adolescent health, very little is known about the extent to which national-level wealth inequalities (i.e., accumulated financial resources) are associated with these inequalities in health. Therefore, this study examined the association between national wealth inequality and income inequality and socioeconomic inequality in adolescents' mental well-being at the aggregated level. Methods: Data were from 17 countries participating in three consecutive waves (2010, 2014, and 2018) of the cross-sectional Health Behaviour in School-aged Children study. We aggregated data on adolescents' life satisfaction, psychological and somatic symptoms, and socioeconomic status (SES) to produce a country-level slope index of inequality and combined it with country-level data on income inequality and wealth inequality (n = 244,771). Time series analyses were performed on a pooled sample of 48 country-year groups. Results: Higher levels of national wealth inequality were associated with fewer average psychological and somatic symptoms, while higher levels of national income inequality were associated with more psychological and somatic symptoms. No associations between either national wealth inequality or income inequality and life satisfaction were found. Smaller differences in somatic symptoms between higher and lower SES groups were found in countries with higher levels of national wealth inequality. In contrast, larger differences in psychological symptoms and life satisfaction (but not somatic symptoms) between higher and lower SES groups were found in countries with higher levels of national income inequality. Conclusions: Although both national wealth and income inequality are associated with socioeconomic inequalities in adolescent mental well-being at the aggregated level, associations are in opposite directions. Social policies aimed at a redistribution of income resources at the national level could decrease socioeconomic inequalities in adolescent mental well-being while further research is warranted to gain a better understanding of the role of national wealth inequality in socioeconomic inequalities in adolescent health.