Background and purpose: The associations of fundamental motor skills (FMS), health -related physical fitness (e.g., cardiorespiratory fitness, CRF), and moderate -vigorous physical activity (MVPA) have been demonstrated in Western children, but these associations have not yet been validated in a sample of Chinese children. The aims of this study, therefore, were to examine the association between FMS subdomains and MVPA in a sample of Chinese children and to evaluate whether this association is mediated by CRF. Methods: A cross-sectional study consisting of 311 children aged 8-12 years (49.2% girls; mean age = 9.9 years) from Shanghai was conducted. FMS, CRF and MVPA were assessed using the Test of Gross Motor Development -3rd Edition, Progressive Aerobic Cardiovascular Endurance Run and ActiGraph GT3X accelerometers. Preacher & Hayes's bootstrap method was used to test the mediating effects of CRF on the association between FMS and MVPA. Results: CRF fully mediated the association between total FMS and MVPA in girls (indirect effects, b = 0.21, 95% CI [0.07-0.37]), while the mediation was only partial in boys (indirect effects, b = 0.12, 95% CI [0.01-0.26]). CRF fully mediated the association between locomotor skills and MVPA in girls (indirect effects, b = 0.27, 95% CI [0.09- 0.51]), whereas CRF partially mediated the association between object control skills and MVPA in boys (indirect effects, b = 0.15, 95% CI [0.18-0.35]). Conclusion: In order to better design and implement sex -specific interventions aiming to increase MVPA, it is essential to consider FMS subdomains and CRF alongside the sex differences in the association between them.