Background: Socio-economic status is one of the most important factors shaping students' motivation and achievement but has seldom been explored in relation to achievement goals. Aims: This study aimed to investigate whether mastery-approach goals explain the link between SES and key learning-related outcomes ("mediation") and whether SES modifies the relationship between mastery-approach goals and these outcomes ("moderation"). Sample: Data came from 595,444 students nested in 21,322 schools across 77 countries. Methods: Data were analysed using multilevel-moderated mediation analyses. Results: We found significant mediation and moderation. In terms of mediation, mastery-approach goals mediated the association between family SES and learning-related outcomes. However, a different pattern emerged for school SES, as students in higher SES schools had lower mastery-approach goals. In terms of moderation, we found that family SES strengthened the association between mastery-approach goals and learning-related outcomes. However, the association between mastery-approach goals and learning-related outcomes was weaker in higher SES schools. Conclusion: Theoretical and practical implications for the achievement goal approach to achievement motivation are discussed.