Action planning is assumed to mediate between intentions and health behaviours. Moreover, intentions are assumed to moderate the planning-behaviour relation, because people with high intentions are more likely to enact their plans. The present studies extend these suppositions by integrating both assumptions to a novel and parsimonious model of moderated mediation: the mediation effect is hypothesised to be stronger in individuals who report higher intention levels. In two longitudinal studies on physical activity (N = 124) and interdental hygiene (N = 209), intentions and action planning were assessed at baseline, and behaviour was measured four (Study 1), and respectively, three (Study 2) months later. The moderated mediation hypothesis was tested with continuously measured intentions using regression analyses with non-parametric bootstrapping. Results from both studies suggest that levels of intentions moderate the mediation process: The strength of the mediated effect increased along with levels of intentions. Planning mediates the intention-behaviour relation, if individuals hold sufficient levels of intentions. Implications for theory advancement and intervention development are discussed.