Objective: Positive self-perceptions of ageing are associated with better health; however, little is known about the potentially underlying mechanisms. The present longitudinal study examines whether the relationship between self-perceptions of ageing and self-rated health is mediated by physical activity in older adults with multiple chronic conditions. Design: A sample of 309 German community-dwelling older adults aged 65-85 years with two or more chronic conditions was assessed at three measurement occasions over 2.5 years. Participants provided information on self-perceptions of ageing, physical activity, self-rated health, number of chronic conditions and demographics (T1). Physical activity was reassessed six months later (T2) and self-rated health after 2.5 years (T3). Data were analysed using multiple regression and path analyses. Main outcome measures: Self-rated health and physical activity. Results: More positive self-perceptions of ageing were related to better self-rated health over a 2.5-year period controlling for confounding variables. Physical activity six months after T1 partially mediated this relationship. Conclusion: Having more positive self-perceptions of ageing is associated with higher levels of physical activity, which in turn predict better self-rated health over time. This supports the hypothesis of a behavioural pathway in the self-perceptions of ageing - health link.