Adolescents living in poorer economic circumstances have an increased risk of mental health problems. Most research investigating this is from high-income countries in Europe and North America. There is less understanding of the determinants of mental health in low- and middle-income countries where the majority of the world’s adolescents live. In my thesis, I investigated the magnitude and the causal mechanisms of economic inequalities in adolescents’ mental health in multiple countries. In study 1, I mapped and categorised causal mechanisms through which adolescents’ economic circumstances might impact their internalising mental health. These were identified through a systematic search of mediation studies and by consulting experts. In study 2, I examined the cross-sectional association between food insecurity and suicidal thoughts and behaviours. The magnitude of the association differed across eighty-three countries and indicators of country-level context, including countries’ economic development. In study 3, I investigated the longitudinal association between adolescents’ economic circumstances and their internalising symptoms, which varied in magnitude across eight countries. Subjective measures of economic circumstances were more strongly associated with future internalising symptoms than objective measures were. Motivated by the lack of mediation studies in low- and middle-income countries, as identified in study 1, I investigated if adolescents’ positive relations with their parents or peers mediated the longitudinal association between their economic circumstances and internalising symptoms in Ethiopia, India, Peru and Vietnam in the final study. I found no evidence of mediation in any country. The magnitude of the association between different measures of economic circumstances and adolescents’ mental health varies across countries. My thesis also highlight the complex causal mechanisms through which economic circumstances affect adolescents’ internalising mental health, and that they may differ across settings. Economic inequalities in adolescents’ mental health are therefore not generalisable across countries and are likely shaped by the wider societal context.