Background <p>Secondary students’ STEM aspirations strongly predict later STEM degrees and careers. Understanding how these aspirations are shaped is therefore critical. Family and school contexts are widely recognized as important influences, but the specific pathways through which these influences operate remain underexplored. Grounded in science capital theory, this study conceptualizes STEM social capital in terms of parental STEM occupations and homeroom teachers’ STEM backgrounds, and examines how these factors relate to Chinese secondary school students’ STEM aspirations, the mediating pathways involved, and heterogeneity across sex and urban–rural contexts.</p> Results <p>Using data from the 2014–2015 follow-up wave of the nationally representative longitudinal data from the China Education Panel Survey (CEPS), this study analyzed a restricted sample of 5,123 eighth-grade students in 122 classrooms across 61 schools. The sample was restricted to schools with random classroom assignment in seventh grade and no performance-based reshuffling in eighth grade to better identify homeroom teacher effects. Balance checks suggest that the restricted analytical sample is broadly comparable to excluded cases on most school-level characteristics, although it includes fewer rural and socioeconomically disadvantaged schools and students. Linear probability models with school fixed effects were estimated while controlling for student, family, and classroom characteristics, followed by mediation and heterogeneity analyses. The results showed that Chinese secondary school students with greater STEM social capital were more likely to report STEM aspirations. Parental STEM occupations influenced aspirations through STEM expectations, with students’ perceived parental STEM expectations exerting a stronger mediating effect. Homeroom teachers’ STEM backgrounds were linked to aspirations through mathematics competency, with mathematics performance showing a slightly stronger mediating effect than students’ self-assessment of mathematics competency. Heterogeneity analyses showed that parental STEM occupations, particularly fathers’ STEM occupations, were more strongly associated with girls’ STEM aspirations, whereas homeroom teachers’ STEM backgrounds were more strongly associated with boys’ STEM aspirations. In addition, parental STEM occupations were significant only in urban schools, whereas homeroom teachers’ STEM backgrounds were more pronounced in rural schools.</p> Conclusions <p>This study provides a relational and contextually grounded account of how multiple sources of STEM social capital shape secondary school students’ STEM aspirations in the Chinese context. By revealing family- and school-based pathways, this study provides practical guidance and insights for parents, teachers, and policymakers on how to support students’ STEM aspirations in China and other similar educational contexts.</p>