Planning is an effective strategy to translate intentions into behavior, and planning interventions can facilitate behavior change. The present study aimed to examine when and how a planning intervention facilitates physical activity. Therefore, the moderating role of intention strength and the mediating role of the specificity of action plans, coping plans, as well as coping self-efficacy were investigated. Furthermore, intention strength was tested as a moderator of the mediation effect. Hypotheses were examined based on a randomized controlled trial (N = 107 students) with 2 groups (planning group vs. control group) and pretest (t1) and posttest (t2) with a 1-week interval. Participants in the planning group increased their physical activity level between t1 and t2 significantly more than those in the control group. The interaction Group × Intention was significant, indicating that intention was a significant predictor of physical activity behavior (t2) not in the planning group but in the control group. Across both groups, the mediation effects were moderated by intention strength. Participants with stronger physical activity intentions showed stronger (action planning and coping self-efficacy) or weaker (coping planning) mediation effects compared with participants with weaker physical activity intentions. In the comprehensive model, only the mediation effect of coping planning was moderated by intention. Our results highlight the moderating role of intention in the context of a planning intervention and revealed that the direct and mediation effects depend on the level of intention strength. Moderated mediation provides a better understanding of when and how a planning intervention works. (PsycINFO Database Record (c) 2019 APA, all rights reserved)