Purpose/Objective: After rehabilitation, it is important to maintain adopted target behaviors such as physical activity and physical exercise. By generating detailed behavioral plans, rehabilitants may translate their intentions into actual behavior. The aim of this study was to investigate whether Conscientiousness would further facilitate this mechanism in a way that rehabilitants who are more conscientious would be more likely to act upon their plans. Research Method/Design: The study presents secondary analyses from a larger intervention in cardiac and orthopedic rehabilitation. N = 136 rehabilitants were surveyed via online questionnaires during treatment (Time 1 [T1]) and with computer-assisted telephone interviews (CATIs) 6 weeks (Time 2 [T2]) and 6 months (Time 3 [T3]) after discharge from the clinic. Intention, planning, Conscientiousness, and behavior were measured. A conditional process analysis examined whether Conscientiousness moderates the intention–planning–physical activity chain. Results: Planning had a mediating effect on intentions and physical activity. Moreover, this mediation effect was conditional on the level of Conscientiousness: Highly conscientious rehabilitants were more successful in translating their plans into behavior than their less conscientious counterparts. Conclusions/Implications: Self-regulatory mechanisms such as action-facilitating planning strategies may be more efficient in conscientious rehabilitants. Conscientiousness reflects dispositional self-regulation and can be supportive when it comes to maintain difficult behaviors. Lack of Conscientiousness needs to be compensated by more intense support of rehabilitation patients, in order to plan appropriately and to act upon these plans in the face of barriers.