Background and objective <p>This study examined the relationship between motor competence (MC) and Physical Activity (PA) in school-aged children, and assessed the mediating role of physical fitness, based on the Model of the Relationship between Children’s Motor Development and Obesity Risk.</p> Methods <p>From March to April 2022, 1,026 children (53.6% boys, mean age 8.93 years) from four public primary schools in Shijiazhuang City, China, were recruited via stratified cluster sampling. MC was assessed using the Test of Gross Motor Development, 3rd edition (TGMD-3), PA was measured via a three-axis accelerometer, and physical fitness was evaluated according to the Chinese National Student Physical Health Standards (2014 revision). Data were analyzed using SPSS 26.0, with mediation tested via the bias-corrected bootstrap method (10,000 resamples).</p> Results <p>Ball skills (<i>r</i> = 0.099, <i>P</i> &lt; 0.01) and composite skills (<i>r</i> = 0.093, <i>P</i> &lt; 0.01) were positively correlated with MVPA. Mediation analysis indicated that physical fitness partially mediated the relationship between ball skills and MVPA (<i>β</i> = 0.020, 95%<i>CI</i> [0.002, 0.044]) as well as the relationship between composite skills and MVPA (<i>β</i> = 0.024, 95% <i>CI</i> [0.002, 0.048]). The mediating effects accounted for 14.8% and 12.8% of the total effects, respectively. In contrast, locomotor skills were not significantly associated with either light physical activity (LPA) or MVPA among school-aged children (<i>P</i>_<i>all</i> &gt; 0.05).</p> Conclusion <p>Ball skills are critical for promoting MVPA in school-aged children, with physical fitness acting as a significant mediator. Systematic ball skill training is recommended as a core strategy to enhance physical activity via improved fitness.</p>