This study aimed to identify differences in individual, social, and physical environmental factors across the stages of physical activity and explore the effect of those variables on physical activity among older adults. Two hundred and ninety adults aged over 65 years were recruited from the Seodaemun district of Seoul. Standardized scales were used to measure physical activity, stage of change, self-efficacy, decisional balance, social support, and the physical environment. The results indicated that 70.7% of older adults engaged in regular physical activity. Based on the multivariate analysis of variance, only the psychological variables of self-efficacy and perceived benefits and the environmental variables of family support and the physical environment increased significantly across the stages of physical activity change. Based on the regression analysis, the most important predictors of physical activity in a descending order of significance were self-efficacy, perceived benefits, family support, and the physical environment. The total variance in physical activity explained by these variables was 40%.