Socioeconomic disadvantage can become ingrained in children’s self-perceptions. These self-perceptions may, in turn, harm children’s academic achievement. Here, we asked: Do children’s self-perceptions help explain socioeconomic disparities in academic achievement around the world? We addressed this question using data from the 2018 Programme for International Student Assessment (PISA) survey, including n = 520,729 records of 15-year-old students from 70 countries. We identified five dimensions of children’s self-perceptions measured in the PISA dataset, i.e., self-perceived competency, self-efficacy, growth mindset, sense of belonging, and fear of failure. As predicted, across countries, children’s self-perceptions jointly and separately partially mediated the association between socioeconomic status and academic achievement. The positive mediation effect of self-perceived competency was more pronounced in countries with higher social mobility and lower income inequality, indicating the importance of environments that “afford” the use of beneficial self-perceptions. By contrast, growth mindset and sense of belonging were more strongly related to achievement in countries with lower social mobility, suggesting that those self-perceptions might become less important for achievement under more supportive external conditions.