Background Physical activity (PA) is important for preventing falls in older age. However, most older people in Iran have not been involved in any exercise program. The aim of this study was to assess the determinants of physical activity to prevent falls using protection motivation theory (PMT) in Iranian older people.Methods The cross-sectional study was conducted in Qom, Iran, from May to October 2018. Three hundred older people were selected from retirement centers via stratified sampling method. Data was collected using the Rapid Assessment of Physical Activity, and PMT constructs scale. Data analysis was performed using multiple linear regression and descriptive statistical analysis.Results The mean (SD) age of the participants was 64.6 (5.5) and the majority were male (77.7%). Level of perceived self-efficacy of participation was low. Perceived costs and perceived rewards were negatively associated with motivation. Furthermore, protection motivation (β=0.38, P<0.0001), fear (β=0.21, P=0.001), and efficiency of response (β=0.13, P=0.026), significantly predict PA behavior. The study model explained approximately 32% of the variance in PA behavior.Conclusion The results indicated that reducing barriers to exercising might increase PA intention. Also, protection motivation and efficiency of response and fear are considered as the strongest predictors of PA behavior among older people. The results can help health care providers to develop appropriate interventions for promoting PA among older people.