PURPOSE: To investigate associations of objectively-measured physical activity (PA) and sedentary time (ST) with adiposity in a predominantly bi-ethnic (South Asian and White British) sample of young children. METHODS: The sample included 333 children aged 11 months to 5 years who provided 526 cross-sectional observations for PA and body composition. Total PA volume (vector magnitude counts per minute (cpm)), daily time at multiple intensity levels (the cumulative time in activity >500 cpm, >1000 cpm, >1500 cpm and so on up to >6000 cpm), and time spent sedentary (<820 cpm), in light PA (820-3907 cpm) and moderate-to-vigorous PA (MVPA: ≥3908 cpm) were estimated with tri-axial accelerometry. Indicators of adiposity included BMI, waist circumference, and the sum of subscapular and triceps skinfold thicknesses. Statistical analyses were performed using multilevel regression and isotemporal substitution models adjusted for confounders. Effect modification by ethnicity was examined. RESULTS: There was no evidence for effect modification by ethnicity (p-interaction≥0.13). In the whole sample, the accumulated time spent above 3500 cpm (i.e. high light-intensity PA) was inversely associated with the sum of skinfolds (β (95% CI) = -0.60 (-1.19 to -0.021) mm per 20 min/d) and the magnitude of association increased dose-dependently with PA intensity (peaking for time spent >6000 cpm: -1.57 (-3.01 to -0.12) mm per 20 min/d). Substitution of 20 min/d of ST with MVPA was associated with a lower sum of skinfolds (-0.77 (-1.46 to -0.084) mm). CONCLUSIONS: High light-intensity PA appears to be beneficial for body composition in young South Asian and White British children, but higher-intensity PA is more advantageous.This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives License 4.0 (CCBY-NC-ND), where it is permissible to download and share the work provided it is properly cited. The work cannot be changed in any way or used commercially without permission from the journal.