Biological maturation may attenuate hypothesised sex differences in children’s physical activity but overall the evidence for this is equivocal. The study investigated how the selection of different physical activity assessment instruments affects the detected relationship between biological maturation and late primary school children’s physical activity. One hundred and seventy five children (97 girls) aged 10.6 ± 0.3 y completed the PAQ-C self-report questionnaire and wore ActiGraph GT1M accelerometers for 5 consecutive days. Maturity status was predicted by estimating attainment of age at peak height velocity. Following initial exploration of sex differences in PAQ-C (t-test) and multiple ActiGraph outcome variables (MANOVA), the influence of maturity status was controlled using ANCOVA and MANCOVA. Unadjusted analyses revealed that boys were significantly more active than girls according to the PAQ-C (p < 0.0001, d = 0.52) and ActiGraph (p < 0.0001, d = 0.36 – 0.72). After controlling for maturity status the differences in PAQ-C scores increased (p = 0.001, d = 0.64), but significant differences disappeared for ActiGraph data (p = 0.36, d = 0.17 – 0.33). The detected relationship between maturity status and late primary school children’s physical activity is dependent on physical activity assessment tool employed, reflecting the different aspects of physical activity captured by the respective measures.