Background <p>Guided by Bronfenbrenner’s ecological systems theory, particularly the process-person-context-time (PPCT) model, this study examined whether teacher-student relationships (TSR), conceptualized as a classroom-context factor, were associated with English-as-a-foreign-language (EFL) achievement through EFL-learning grit as a person-level motivational resource and behavioral engagement as a proximal learning process. Evidence for this integrated mechanism remains limited in Chinese secondary EFL settings.</p> Methods <p>Participants were 718 Grade 7–8 EFL learners in Chinese secondary schools. Using domain-specific measures and standardized municipal English test scores, we tested a serial mediation model with confirmatory factor analysis (CFA) and structural equation modeling (SEM) while controlling for age, gender, and socio-economic status (SES).</p> Results <p>TSR was positively associated with EFL-learning grit and behavioral engagement. EFL-learning grit was positively associated with behavioral engagement, and behavioral engagement was positively associated with EFL achievement. Indirect effects supported mediation via behavioral engagement and through the serial pathway (TSR→ grit→ behavioral engagement→ EFL achievement), whereas the grit-only indirect pathway was not significant.</p> Conclusion <p>This study contributes to EFL and educational psychology research by integrating contextual, personal, and process factors within a PPCT-informed model, by using domain-specific constructs and an objective achievement indicator, and by showing that TSR is associated with achievement primarily through engagement-related processes rather than through a direct pathway. The findings extend evidence from Chinese secondary EFL classrooms and suggest that relationship-centered teaching practices that strengthen perseverance and sustained participation may support achievement, although causal inferences remain limited.</p>