Objectives. Evidence exists that independently of physical activity, a dose-response relationship exists between sedentary time and adverse health outcomes. However, little is known about motivations underlying sedentary behavior. The purpose of this study was to (i) examine the factor structure and composition of sedentary derived autonomous (identified and intrinsic) and controlled (external and introjected) motives within an Organismic Integration Theory (OIT) framework and (ii) determine whether these motivational constructs are related with overall sitting time as well as sitting for work/school and recreation/leisure on weekdays and weekends. Method. University students or staff (n = 571) completed an internet-based survey within a cross-sectional design. After completing a modified Sedentary Behavior Questionnaire, participants were randomized to one of five groups (general, weekday work/school, weekday recreation/leisure, weekend work/school, weekend recreation/leisure) and completed a sedentary derived 15-item modified Behavioral Regulation in Exercise Questionnaire (BREQ). Results. Factor analysis findings support the tenability of a four-factor model for weekday work/school, weekend work/school, and weekend leisure/recreation sedentary behavior and a three-factor model for general and weekday leisure/recreation behavior. Regression analyses showed the motivational constructs explained a significant amount of sedentary behavior variance for weekend work/school (10%), weekend leisure/recreation (9%), weekday work/school (4%), and weekday leisure/recreation (3%). General sedentary behavior was unrelated with the motivational constructs. In general, autonomous motives underlied leisure/recreational sitting while controlled motives were more strongly associated with work/school behavior. Conclusions. Our findings support the hypothesis that motivational constructs grounded in OIT have the potential to further our understanding of sedentary behavior.