Objective To examine the joint association of birth weight and physical activity/sedentary time with childhood obesity in 12 countries. Methods A cross‐sectional study of 5,088 children aged 9 to 11 years was conducted. Birth weight was recalled by parents or guardians. Moderate‐to‐vigorous physical activity (MVPA) and sedentary behavior were objectively measured using accelerometry. Results The association of birth weight with the odds of obesity, central obesity, and high body fat was significant among children with either low MVPA or high sedentary time but not among children with either high MVPA or low sedentary time. In comparison with children with normal birth weight and high MVPA, children with high birth weight and low MVPA showed 4.48‐ to 5.18‐fold higher odds of obesity, central obesity, and high body fat; children with normal birth weight and low MVPA showed 3.00‐ to 3.30‐fold higher odds of obesity, central obesity, and high body fat, and children with high birth weight and high MVPA showed 1.16‐ to 1.68‐fold higher odds of obesity, central obesity, and high body fat. Conclusions High MVPA is more important than high birth weight as a correlate of obesity in children.