Background: Physical activities are essential to the quality of life of people with intellectual and developmental disabilities. To promote participation in physical activities, it is critical to identify changeable variables based on the socio-environmental approach. The purpose of this study is to explore the status of physical activity participation and its correlate for people with intellectual and developmental disabilities in South Korea. Methods: A total of 416 people with intellectual and developmental disabilities (or their families) completed the Korean national survey. We examined (a) physical activity frequency, duration, types, and facilities; (b) correlates of physical activity participation; and (c) barriers to physical activity participation. Results: Slightly over half of the respondents participated in regular physical activity in the past year almost every day for approximately an hour, mainly by walking and jogging. Hierarchical binary logistic regressions revealed positive correlations between physical activity participation and healthy eating habits, reduced depression and suicidal ideas, community outings, direct professional support, and less feelings of discrimination. Conclusions: To promote physical activity participation among people with disabilities, personal and socioenvironmental variables should be considered.