Objective: This 4-year longitudinal study examined the different roles of perceived stress in the association between older adults' physical activities and physical health. We hypothesized that physical activities would exert beneficial effects on physical health by preventing chronically high levels of perceived stress. Methods: We assessed baseline levels of physical activities and repeated measures of perceived stress and physical symptoms in 3 waves of data from a sample of 157 older adults. Results: Among participants with high (but not low) baseline levels of perceived stress, physical activity predicted a 2-year reduction of perceived stress and a 4-year prevention of physical health symptoms. Moreover, the interaction effect on 4-year changes in physical symptoms was mediated by 2-year changes in perceived stress. Conclusions: Physical health benefits of physical activity are particularly pronounced among older adults who perceive high levels of stress, and this effect is mediated by a prevention of chronically high perceptions of stress.