Early life physical activity may help prevent obesity but is difficult to measure. The purpose of this study was to examine associations of age of achievement of gross motor milestones in infancy with adiposity at age 3 years. Seven forty one mother/infant dyads participated in a longitudinal study in Massachusetts. Exposures were age of attainment of 4 gross motor milestones-rolling over, sitting up, crawling, and walking. Outcomes were 3-year sum of subscapular and triceps skinfold thickness (SS + TR) for overall adiposity, their ratio (SS:TR) for central adiposity, and body mass index (BMI) z-score. We used linear regression models adjusted for confounders to examine motor milestone achievement and later adiposity. Rolling over (0.04, 95% CI: 0.008, 0.07) and sitting up (0.02, 95% CI: 0.001, 0.05) at ≥ 6 months were associated with increased SS:TR compared with attainment before 6 months. Walking at ≥ 15 months was associated with 0.98 mm higher SS + TR (95% CI: 0.05, 1.91) compared with walking before 12 months. Age at crawling was not associated with the outcomes. None of the milestones were associated with BMI z-score. Age of motor milestone achievement was only a modest predictor of adiposity. Later rolling over and sitting up were associated with greater central adiposity, and later age at walking was associated with greater overall adiposity at age 3 years. Although we controlled for birth weight and 6-month weight-for-length in our models, more detailed assessment of early adiposity prior to achievement of motor milestones is needed to help determine causality.