To investigate associations between timing of adiposity rebound (AR; the period in childhood where BMI begins to increase from its nadir) and adiposity (BMI, fat mass) at age 15 years in the Avon Longitudinal Study of Parents and Children (ALSPAC).The sample consisted of 546 children with AR derived in childhood and BMI and fat mass index (FMI; fat mass measured by dual-energy radiograph absorptiometry/height in m(2)) measured at 15 years. Multivariable linear regression models were based on standardized residuals of log BMI and log FMI to allow comparison of regression coefficients across outcomes.There were strong dose-response associations between timing of AR and both adiposity outcomes at 15 years independent of confounders. BMI was markedly higher in adolescence for those with very early AR (by 3.5 years; β = 0.70; 95% confidence interval [CI]: 0.33-1.07; P ≤ .001) and was also higher for those with early AR (between 3.5 and 5 years; β = 0.34; 95% CI: 0.08-0.59; P = .009) compared with those with later AR (>5 years) after full adjustment for a range of potential confounders. Similar magnitudes of association were found for FMI after full adjustment for confounders (compared with later AR: very early AR β = 0.74; 95% CI: 0.34-1.15; P ≤ .001; early AR β = 0.35; 95% CI: 0.07-0.63; P = .02).Early AR is strongly associated with increased BMI and FMI in adolescence. Preventive interventions should consider targeting modifiable factors in early childhood to delay timing of AR.