This study investigated a reciprocal effects model (REM) of children's body fat self-concept and physical self-concept, and objectively measured school physical activity at different intensities. Grade four students (N = 376; M age = 9.07, SD =.61; 55% boys) from the midwest region of the United States completed measures of physical self-concept and body fat self-concept, and wore accelerometers for three consecutive school days at the beginning and end of one school year. Findings from structural equation modeling analyses did not support reciprocal effects. However, children's body fat self-concept predicted future physical self-concept and moderate-to-vigorous physical activity (MVPA). Multigroup analyses explored the moderating role of weight status, sex, ethnicity, and sex*ethnicity within the REM. Findings supported invariance, suggesting that the observed relations were generalizable for these children across demographic groups. Links between body fat self-concept and future physical self-concept and MVPA highlight self-enhancing effects that can promote children's health and well-being.