Background: Research has shown that students from socially disadvantaged backgrounds are more likely to have lower academic performance compared to children from better-off families. However, the way that socioeconomic status (SES) influences student learning outcomes has not yet been fully examined. Thus, this study explores the indirect effects of students' SES on reading achievement through academic motivation. Specifically, the mediating role of the following three motivation elements is investigated: "mastery-approach orientation of achievement goals," "enjoyment of reading," and "expected occupational status." Methods: Structural Equation Modelling (SEM) was used to test the hypothesized mediation model in each European country (n = 38) that took part in the PISA 2018 cycle. Specifically, data was obtained from 237,366 15-year-old students. Common measures for SES, academic motivation, and reading achievement were employed to examine whether the selected academic motivation elements have similar effects in each country. Confirmatory Factor Analysis (CFA) was used to validate the factorial structure of mediators and test their measurement invariance across the samples. Results: Within-country SEM analysis revealed that "expected occupational status" partially mediates the relationship between SES and reading achievement in all countries. "Enjoyment of reading" fitted well as a mediator in almost all countries (n = 37), whereas "mastery-approach orientation of achievement goals" only appeared as a mediator in 11 countries. The direct effect of SES on academic achievement was significantly reduced in all models. Conclusions: This study provides evidence that "enjoyment of reading" and "expected occupational status" can be treated as mediators in the relationship between SES and reading achievement in European countries. Implications for research and theory for promoting equity in education are drawn. It is argued that schools and teachers should encourage students to participate in activities that foster their intrinsic motivation towards reading and raise their educational and occupational expectations, especially for those coming from socially disadvantaged backgrounds.