ABSTRACTImportanceChildhood and adolescence are critical periods for lifelong bone mineral accrual, but few studies have determined the impact of childhood adiposity on adult bone density.ObjectiveTo determine the long‐term impact of childhood adiposity on adult areal bone mineral density (aBMD) and the effect of adult adiposity on this relationship.MethodsWe conducted a longitudinal study of 1156 adults (56.3% men), for whom skinfold thickness (SFT) had been measured during childhood (6–18 years) and fat mass percentage (FMP) and aBMD were measured during adulthood (29–43 years). Adult aBMD in the lumbar spine (LS), femoral neck (FN), arms, and legs was measured using dual‐energy X‐ray absorptiometry. The direct effect of childhood SFT and its indirect effect through adult FMP on adult aBMD were estimated using general linear regression and a causal steps approach.ResultsSignificant positive associations between childhood SFT and adult aBMD were found in the LS in men (β = 0.089, P = 0.044) and in all the skeletal sites in women. With respect to the adult fat–bone relationship, high adult FMP was associated with low aBMD in most of the sites in men, but with high FN aBMD in women (β = 0.144, P = 0.002). Moreover, suppressive effects of adult FMP on the associations between childhood SFT and adult aBMD in the LS (−34.8%) and legs (−67.1%) of men, and a positive effect on the FN aBMD in women (17.0%) were identified.InterpretationChildhood adiposity appears to have a positive long‐term effect on adult aBMD, which may be reduced by adiposity in adult men but reinforced by adiposity in adult women.