Background: Socioeconomic status (SES) and motivation are both important predictors of student achievement. However, most studies have investigated these factors separately, and very few have looked into the interplay between SES and motivation as determinants of student reading achievement. Aims: We intend to bridge this gap by examining a model of SES predicting reading achievement through motivation (i.e., expectancy and value) at both student and school levels. Sample: We used the data from the Programme for International Student Assessment (PISA) 2018 of 26,281 students from four regions in Greater China (Mainland China, Hong Kong, Macau, and Taipei). Methods: We used multi-group multilevel path analysis to test whether SES would predict reading achievement mediated by expectancy and value in student and school levels across four regions, with gender as a covariate. Results: Results showed that at the student level, SES significantly predicted reading achievement indirectly through both expectancy and value across four regions. At the school level, the relationship between school SES and school reading achievement was mostly direct. Conclusion: The study was able to demonstrate the motivational gap as a pathway in which economic inequality can contribute to students' reading achievement gap.