The theory of planned behavior (TPB) has been criticized for not including interaction effects between its major constructs and for not dealing with the processes involved in the intentions–physical activity (PA) relationship. This study tested an extended TPB model including interaction effects between TPB constructs (i.e., attitudes, subjective norms, perceived behavioral control [PBC], and intentions) and additional variables hypothesized to be moderators of the intentions–PA relationship (i.e., planning and perceived built environment). TPB variables, planning, perceived built environment, and PA were measured at Time 1 (N = 612). PA measure was repeated 3 months later at Time 2 (N = 219). Two hierarchical linear regressions were performed to examine the determinants of intentions at T1 and the determinants of PA at T2. Analyses demonstrated significant Attitudes × PBC and Subjective Norms × PBC interactions on intentions (ps < .05). A high level of subjective norms and attitudes strengthen the relationship between PBC and intentions. Analyses also highlighted a significant Intentions × Planning × Perceived Built Environment triple interaction on PA (p < .05). The relationship between intentions and PA is only significant for participants with a medium or high level of planning and a high perceived proximity to PA facilities. Future studies should test interaction effects between TPB variables and explore the interplay between psychosocial and environmental variables when focusing on the intentions–PA relationship.