Abstract Background The study aimed to examine the cross-sectional associations between 24-h activity compositions and motor competence in children and adolescents, while stratifying by sex and school type (primary or secondary school) and estimate differences in motor competence associated with reallocations of time between activity behaviours. Methods Data were collected from 359 participants (aged 11.5 ± 1.4 years; 49.3% boys; 96.9% White British). Seven-day 24-h activity behaviours [sleep, sedentary time, light physical activity (LPA), moderate-to-vigorous physical activity (MVPA)] were assessed using wrist-worn accelerometers. Motor competence outcomes were obtained using the Dragon Challenge (process, product, time, and overall scores). Linear mixed models examined associations between activity behaviour compositions and motor competence outcomes for all participants and stratified by school type (primary or secondary) and sex. Post-hoc analyses modelled the associations of reallocating fixed durations of time between activity behaviours with the outcomes. Results In all participants, relative to other activity behaviours, MVPA had the strongest associations with motor competence outcomes. Time reallocations (5, 10, 15, 20 min) to MVPA from any of the other three behaviours were associated with higher overall, process, and time scores [effect sizes (ES) = 0.05–0.07 (5 min) and 0.19–0.27 (20 min)]. The stratified models displayed that MVPA had the strongest associations with outcomes in both sexes, irrespective of school type. The largest positive, and negative estimated differences occurred when MVPA hypothetically replaced LPA or sleep [ES = 0.04–0.10 (5 min) and 0.14–0.39 (20 min)], and when LPA or sleep hypothetically replaced MVPA [ES = − 0.03 to − 0.11 (5 min) and − 0.13 to − 0.54 (20 min)], respectively. Conclusions Relative to other activity behaviours, MVPA had the strongest association overall with motor competence outcomes. Hypothetical reallocations of time from LPA or sleep to MVPA (and vice versa) were associated with the largest positive estimated differences in motor competence outcomes. Therefore, our findings reinforce the key role of MVPA for children’s and adolescents’ motor competence.