Physical activity and sport settings may become socially threatening when students anticipate negative evaluation of their bodies, competence, or behavior. Yet it remains unclear whether social interaction anxiety precedes exercise-related avoidance and whether such avoidance, in turn, precedes later anxiety. This study examined reciprocal longitudinal associations at both the between- and within-person levels between social interaction anxiety (SIA) and the tendency to avoid physical activity and sport (TAPAS), assessed with a 10-item self-report scale. A baseline cohort of 1,676 university students aged 18 years or older from nine universities in Sichuan Province, China, completed four assessments approximately 12 weeks apart during the 2024–2025 academic year. A random-intercept cross-lagged panel model separated stable between-person differences from time-specific within-person deviations, and longitudinal scalar invariance was supported for both measures. At the between-person level, higher average SIA was associated with higher average TAPAS (r = 0.381, p < 0.001). At the within-person level, elevations in SIA relative to students’ usual levels predicted subsequent elevations in TAPAS (βs = 0.121–0.127), and elevations in TAPAS predicted subsequent elevations in SIA (βs = 0.126–0.133); all cross-lagged effects were significant at p < 0.001. Both constructs also showed significant within-person temporal continuity. The reciprocal pattern was stable across the three approximately 12-week intervals and remained substantively unchanged after adjustment for baseline sex, year of study, pre-university residence, and body mass index category. The findings are consistent with a reciprocal avoidance process across an academic year and suggest that campus mental-health and physical-activity initiatives may benefit from jointly addressing social-evaluative concerns and barriers embedded in exercise settings. Because the study was observational and relied on self-report measures, the associations should not be interpreted causally.