Objective: This study tested the hypothesized stress-buffering effects of social support on physical activity, sitting time, and blood lipid profiles. Participants: 537 college students. Methods: College students volunteered to self-report stress, social support for exercise, physical activity and sitting time, and provided blood samples to assess lipid profiles in this cross-sectional study. Results: Lower stress was associated with higher vigorous physical activity ([beta] = -0.1, t = -2.9, p = 0.004). Higher social support was associated with higher moderate ([beta] = 0.2, t = 2.0, p = 0.042), vigorous ([beta] = 0.5, t = 5.4, p < 0.001), and total ([beta] = 0.1, t = 3.2, p = 0.001) physical activity, and lower sitting time on weekdays ([beta] = -0.1, t = -3.3, p = 0.001) and weekends ([beta] = -0.2, t = -3.6, p < 0.001). Social support moderated the association between stress and sitting time on weekdays. Conclusions: Stress reduction and fostering social support may be important strategies for promoting physical activity and reducing sedentary behaviors in college students. Additional strategies are needed to buffer the deleterious effects of stress.