<p>This study investigates the relationship between project-based learning and mathematics achievement by focusing on the mediating roles of attitudes towards cooperation and motivation. Using data from a large-scale survey of 17,364 eighth-grade students across 378 schools in China, this study reveals the following key findings: (1) opportunities for project-based learning significantly influence both students’ attitudes towards cooperation and their motivation; (2) motivation plays a key mediating role in the relationship between opportunities for project-based learning and mathematics achievement; and (3) attitudes towards cooperation sequentially impact motivation, which in turn affects mathematics achievement. These results highlight the importance of fostering a collaborative and motivating learning environment in project-based learning. Implications for mathematics educators are discussed, and suggestions for future research are provided to explore the longitudinal effects and additional contextual factors that influence the impact of project-based learning on student achievement.</p>