Objective: To investigate physical activity among older people living alone in Shanghai, People's Republic of China, and key factors contributing to their physical activity. Methods: A cross-sectional questionnaire survey was administered in nine communities in Shanghai, using a stratified random cluster sample: 521 community-dwelling older people living alone (aged 60 years and above) completed a structured questionnaire through face-to-face interviews. Their physical activity was measured using their self-reported frequency of engaging in different types of activities. Other data collected included self-rated health, number of chronic diseases, depression, functional ability, loneliness, social support and socio-demographic variables. Results: Only 16.7% of participants reported adequate physical activity, defined as performing at least 30 minutes of moderate/strenuous exercise on at least 5 days per week. A younger age, a lower level of education, better functional ability and walking outside the home 5-7 days per week increased the likelihood of adequate physical activity. Gender, self-rated health, number of chronic diseases, depression and social support also had relationships with physical activity levels. Conclusion: The prevalence of adequate physical activity in older people living alone in Shanghai was low. In this population, appropriate health education interventions to promote physical activity levels are needed.