BACKGROUND Previous studies have reported on the associations between obesity and sedentary behavior (SB) or physical activity (PA) in children. This study examined longitudinal and bidirectional associations between adiposity and SB and PA in children. METHODS Participants were 356 children in England. PA was measured at 7 and 9 years of age using accelerometry. Outcome and exposures were time in SB and PAs and concurrent body mass index z score and fat index (FI). RESULTS Adiposity at baseline was positively associated with changes in SB (β = 0.975 for FI) and negatively associated with changes in moderate to vigorous PA (β = -0.285 for body mass index z score, β = -0.607 for FI), vigorous PA (β = -0.095 for FI), and total PA (β = -48.675 for FI), but not vice versa. The changes in SB, moderate to vigorous PA, and total PA for children with overweight/obesity were significantly more adverse than those for children with healthy weight. CONCLUSIONS A high body mass index z score or high body fatness at baseline was associated with lower moderate to vigorous PA and vigorous PA after 2 years, but not vice versa, which suggests that in this cohort adiposity influenced PA and SB, but the associations between adiposity and SB or PA were not bidirectional.