Parent and child physical activity levels are correlated, but are they interdependent? A dyadic version of the theory of planned behavior (TPB) was developed to investigate interdependence in the motivation and intention of parents and their children with overweight to engage in healthy physical coactivity (HPCA). Baseline measures of the TPB constructs (subjective norms, attitude, perceived behavior control, and intention) for both dyad members were used to predict parent—reports of their actual HPCA at 12 weeks using the actor–partner interdependence model. The sample included 65 mother–child dyads and 48 father–child dyads from 66 predominantly Caucasian families. In mother–child dyads, a positive attitude toward HPCA predicted each person’s own intention to engage in HPCA (both actor effects). In addition, mother’s perceived behavior control over HPCA predicted the child’s intention to engage in HPCA (a partner effect). Mother’s attitude toward HPCA also predicted mother-reported HPCA. In father–child dyads, perceived behavior control predicted each person’s own intention to engage in HPCA (both actor effects). The child’s intention was also predicted by the child’s subjective norms (an actor effect) and the father’s perceived behavior control (a partner effect). Only the child’s perceived behavior control predicted father-reported HPCA. There is interdependence in the motivation to engage in HPCA because both parents’ perceived behavior control predicted their child’s intention. However, interventions targeting mother’s attitude toward HPCA with her child and the child’s perceived behavior control in relation to the father would be most likely to increase HPCA in the parent–child dyads of children with overweight.