Theoretically grounded research on the determinants of Mexican American children's physical activity and related psychosocial variables is scarce. Thus, the purpose of our investigation was to evaluate the ability of the theory of planned behavior (TPB) to predict Mexican American children's self-reported moderate-to-vigorous physical activity (MVPA). Children (N = 475, ages 9-12) completed questionnaires assessing the TPB constructs and MVPA. Multiple regression analyses provided moderate support for the ability of the TPB variables to predict MVPA as we accounted for between 8-9% of the variance in MVPA. Attitude, subjective norm, and perceived behavioral control accounted for 45% of the variance in intention. Descriptive results were encouraging because mean values indicated that most children had positive attitudes, moderately strong intentions, felt in control, and perceived support from significant others (i.e., physical education teachers) for their physical activity engagement.