Motor competence (MC) evolves throughout childhood as biological maturation progresses, yet previous research has largely approached this relationship linearly. Whether more complex patterns exist remains unclear, and while higher body mass index (BMI) is associated with lower MC, its contribution to maturityrelated differences is undetermined. This study examined the relationship between biological maturation and basic MC (BMC) and explored whether BMI mediates this association in schoolchildren. A total of 1155 Italian children (665 girls; age = 9.8±2.0 years) completed anthropometric measurements and the MOBAK assessment battery during physical education classes. Biological maturation was estimated using maturity offset equations and expressed as years from peak height velocity (PHV). Quadratic regression models were used to test the association between maturation and BMC. Mediation analysis was conducted to determine whether BMI z-score mediated the relationship between maturation and BMC. Quadratic modeling demonstrated that maturation was non-linearly associated with BMC (β = 0.279, p < 0.001), indicating a U-shaped trajectory, with lower competence observed near PHV and higher values pre- and post-PHV. BMI z-score was negatively associated with BMC (β = -0.196 to -0.220, p < 0.001) but did not mediate the maturation-BMC relationship. Sex differences emerged by domain, with girls outperforming boys in self-movement tasks and boys scoring higher in object-movement tasks. Maturation shows a non-linear association with BMC, characterized by a temporary decline around PHV. Although BMI is negatively related to BMC, it does not explain maturation effects. These findings highlight PHV as a sensitive developmental period during which targeted motor skill support may be needed in educational and sport settings.