The purpose of this study was to explore the role of habit in predicting physical activity with the theory of planned behavior (TPB). The study extended previous research by (a) including a measure of temporal intention stability in the regression equation, and (b) unpacking the intention x behavior x habit relationship. Participants were 153 undergraduate students who completed a habit measure and measures of the TPB at Time 1 followed by measures of intention and behavior 2 weeks later. Results using regression analysis demonstrated that habit explained 7% additional variance after accounting for the TPB and temporal stability of intention and its interaction with intention. Follow-up analyses showed considerable asymmetry in the three-way relationship between intention, behavior, and habit, where high habit participants were composed primarily of intenders (i.e., intended to be active >3 times/week at 30 min) who engaged in regular physical activity (70%, n = 28) and low habit participants were inactive nonintenders (i.e., did not intend to be active >3 times/week at 30 min and were subsequently not active; 69%, n = 25). The results support the notion that some properties of physical activity may have an automatic component and that habits may be important to physical activity action control.