Retirement has been identified as a life transition that is important in determining older adults’ physical activity levels. The present study examined the factors associated with retirement that affect physical activity participation among older adults. Semistructured interviews were conducted with 425 retired Australians aged 60 years and older. Physical activity was assessed objectively, using accelerometers. Two categories of factors affecting physical activity participation following retirement were identified: the various physical and psychological changes in later life that can encourage or discourage physical activity and the adaptation processes undertaken by older people in response to these changes. The adoption of either a gain or loss approach to retirement and aging appeared to be the most influential adaptation factor affecting physical activity participation. The results suggest that intervention approaches should aim to foster more positive attitudes to aging and retirement and promote physical activity at all stages in life.