Academic Stress, Subsequent Physical Activity, and Depressive and Anxiety Symptoms Among College Students: A Three-Wave Longitudinal Study

College students’ physical activity may decline when academic demands are high, even among those who previously intended to exercise. This three-wave longitudinal study examined whether T2 physical activity, modeled conditional on T1 exercise intention and T1 physical activity, was involved in a time-ordered indirect association between T1 academic stress and T3 depressive and anxiety symptoms. Of 982 students who completed the baseline survey in September 2025, 750 provided complete matched data in December 2025 and March 2026 (retention = 76.4%). Primary models used raw T2 physical activity and adjusted all component, direct, and total-effect equations for the identical covariate set: gender, age, BMI, T1 exercise intention, T1 physical activity, T1 PHQ-9, and T1 GAD-7. A sign-reversed residualized activity score was examined only as an algebraically equivalent parameterization. T1 exercise intention predicted higher T2 physical activity (β = 0.308), whereas T1 academic stress was associated with lower T2 physical activity (B = −12.771, p < 0.001). Under identical adjustment, raw T2 physical activity was inversely associated with T3 depressive symptoms (B = −0.0024) and anxiety symptoms (B = −0.0013); the corresponding sign-reversed residualized coefficients were equal in magnitude and opposite in sign, with identical R2 values (0.4257 and 0.2720). Full-process bootstrap indirect associations were significant for depressive symptoms (B = 0.0309, 95% CI [0.0229, 0.0396]) and anxiety symptoms (B = 0.0162, 95% CI [0.0099, 0.0236]). The Stress × Intention interaction and quadratic residualized-activity terms were not significant. No observed T1 variable differed significantly between retained and attrited participants. Higher academic stress was associated with relatively lower subsequent physical activity, which was in turn statistically associated with later depressive and anxiety symptoms. The findings do not establish causality, disruption of intention enactment, or a distinct psychological residual mechanism.