Background Fostering self‐efficacy and planning in individuals can support the uptake and maintenance of regular physical activity. This study examined self‐efficacy and planning as mechanisms of an online‐delivered workplace health promotion intervention to enhance employees' physical activity. A special focus lay on reciprocal interrelations among self‐efficacy and planning over time, as previous work predominantly accounted for only one predictive direction at a time. Methods Data from N = 1,063 employees of a pharmaceutical company who reported an intention to increase their physical activity levels were assessed at three measurement points up to 12 weeks following the intervention. Cross‐lagged panel analyses were performed to examine effects of self‐efficacy and planning on physical activity as well as reciprocal interrelations between self‐efficacy and planning. Results Findings indicated an increase in self‐efficacy, planning, and physical activity following the intervention. Planning was consistently linked to subsequent physical activity, whereas self‐efficacy was not associated. Also, reciprocal interrelations among self‐efficacy and planning were found across both measurement lags. Conclusions Planning was confirmed as a predictor of physical activity, whereas self‐efficacy was not. However, cross‐lagged interrelations indicated reciprocal reactivation among self‐efficacy and planning over time, suggesting beneficial effects of including strategies that foster both volitional constructs in interventions.