Prospective memory (PM) is the ability to remember to perform an intended action in the future and is necessary for regular physical activity (PA). For older adults with declining PM, planning strategies may help them to act upon their intentions. This study investigates PM as a moderator in a mediation process: intention predicting PA via planning. A mediated moderation was estimated with longitudinal data of older adults (M = 70 years). Intentions (T1) predicted PA (T3) via action and coping planning (T2). PM was included as moderator on the planning-PA association. Both planning strategies were significant partial mediators (action planning: b = 0.17, 95 % CI [0.10, 0.29]; coping planning: b = 0.08, 95 % CI [0.02, 0.18]). For individuals with lower PM, the indirect effect via coping planning was stronger than with higher PM (b = 0.06, 95 % CI [0.01, 0.16]). Action planning is important for PA in old age regardless of PM performance, whereas older adults with lower PM benefitted most from coping planning. Intervention studies for older adults should consider training PM and promote planning skills.