Background: Early adiposity rebound (AR) is a key predictor of later obesity and metabolic risk, yet modifiable factors related to early AR remain understudied in large populations.Purpose: To quantify the prevalence of early AR at age 3 years and identify modifiable correlates in a population‑based cohort of Japanese preschool children.Methods: We retrospectively analyzed health-check records for 74,466 children who attended both 1.5- and 3-year examinations (2014–2019). Body mass index (BMI) values were converted to World Health Organization z scores; early AR was defined as any increase in BMI between 1.5 and 3 years. Multivariable logistic regression adjusted for birth weight category, sex, household structure, sleep duration, and behavioral factors.Results: Early AR occurred in 18,673 children (25%), whereas obesity (BMI z score ≥1.64) was present in 4% at 3 years. After controlling the adjustments, routine breakfast consumption (odds ratio [OR] 0.88; 95% confidence interval [CI], 0.81–0.97) and regular napping at 1.5 years (OR, 0.84; 95% CI, 0.79–0.90) were independently associated with reduced odds of early AR, while obesity at 1.5 years strongly predicted early AR (OR, 4.32; 95% CI, 4.00–4.67). Routine juice intake or fast-food consumption showed no significant associations.Conclusion: In this population‑based cohort, one in 4 preschoolers had early AR by age 3. Simple daily routines—eating breakfast and maintaining regular sleep—may help delay AR and offer actionable targets for early obesity prevention.