Background: The main concern in many countries is to increase physical activity levels of adolescent populations in terms of prevention of obesity and adult disease. Physical activity levels of university students in South Korea have decreased remarkably in recent years. The motivation process has been regarded as a key factor for the understanding of participation and maintenance in physical activity. As a research gap discovered in previous studies, the exploration of motivational processes including multiple factors is helpful to find out the precise reason students engage in physical activity, then improving their physical activity levels. In particular, this study focused on the mediating role of action planning to explore the effects on physical activity behavior drawn by motivational tenets of Self-Determination Theory and Planned Behavior of Theory. Purpose: The purpose of this study examined the relationships between autonomous motivation, behavioral intention, action planning, and physical activity behavior using the tenets of Self-Determination Theory and Planned Behavior of Theory. The study hypothesized that action planning mediates the relationship between behavioral intention and physical activity behavior being moderated by autonomous motivation. Method: A total of 203 students in two universities located at Daejeon city in South Korea participated in physical education classes for one semester. The participants completed four well-validated questionnaires at the end of the semester. To test the hypothesized model, structural equation modeling (SEM) analysis was used. Findings and conclusions: A moderated mediation model indicated that action planning mediated the relationship between behavioral intention and physical activity behavior through its mediation effect. The results also revealed autonomous motivation self-regulated by students facilitated the transfer of behavioral intention into physical activity behavior through action planning. The findings demonstrated that action planning played as an effective tool to occur physical activity behaviors via behavioral intention moderated by autonomous motivation and provided useful strategies as an intervention to increase physical activity levels of university students in South Korea.