["Journal of Child Psychology and Psychiatry, EarlyView. ", "\n\nBackground\nAdolescents frequently experience psychosomatic comorbidities, yet longitudinal, bidirectional relationships among physical activity (PA), social competence (SC), and psychosomatic comorbidity (PC: mental health and sleep duration) remain underexplored in large non‐Western samples. This study examined how PA and SC mutually influence PC over time and tested whether SC mediates the PA‐PC link in Chinese high school students.\n\n\nMethods\nData came from the National Adolescent Health Survey (China) collected from 2020 to 2023, matching participants by name at one‐year intervals. The analytic sample (n = 9,680; 51.2% female, mean age = 15.89 ± 1.06 years) completed validated measures. We tested measurement invariance, then estimated a two‐wave cross‐lagged panel model controlling for age, gender, residence, and only‐child status. SC mediation was assessed via the product of cross‐lagged paths with 1,000‐sample bootstrap. Multiple‐group SEM examined gender invariance.\n\n\nResults\nAll measurement models achieved scalar invariance. Cross‐lagged paths indicated PA₁ → SC2 (β = 0.041, p < .001), PA₁ → SD2 (β = 0.018, p < .05), and PA₁ → MH2 (β = −0.023, p < .05). SC₁ → MH2 (β = −0.034, p < .01). MH₁ → PA2 (β = −0.038, p < .001), and SD₁ → PA2 (β = 0.014, p < .05). Indirect effect PA₁ → SC2 → PC2 was significant (standardized = −0.001; 95% CI [−0.003, −0.001]). Gender group comparison showed no significant differences (Δχ2 (5) = 8.10, p = .16).\n\n\nConclusions\nPA and PC exert reciprocal influences in adolescents, with higher PA predicting improved SC, better SD, and reduced MH, while psychosomatic distress undermines subsequent PA and SC. SC partially mediates PA's protective effects on comorbidity, with equivalent patterns across genders. Interventions combining PA and social‐skills training may disrupt the vicious cycle of adolescent PC.\n\n"]