This study adopted a three-wave random intercept cross-lagged panel model (RI-CLPM) to explore the longitudinal reciprocal relationships between social support and engagement in EFL learning among Chinese high school students. Two samples, including 3505 students (Sample 1 = 1794; Sample 2 = 1711), were tracked with their perceived social support and engagement levels in EFL learning over three academic semesters. Using a multiple-indicator RI-CLPM, the study demonstrated a reciprocal relationship between perceived social support and engagement in EFL learning maintained consistently across samples. Perceived social support in EFL learning positively predicted students' subsequent learning engagement. Meanwhile, students' engagement in EFL learning positively predicted their subsequent perceived social support. While prior cross-sectional research mostly showed how social support predicts student engagement, not the reverse prediction, the current study provides initial evidence for the possibly mutually reinforcing relationship over time.