Objective: To examine the association of perceived stress [PS] with physical activity [PA] and sitting time [ST], before and during COVID-19 among US college students. Participants: 592 students from a large Western university (19.08 ± 4.62 years, 76.69% female, 79.39% White) were recruited. Methods: Self-reported online survey was completed in June-December 2020. Analysis included paired T-tests for continuous variables, Pearson Correlation the relationship between PSS, PA, and sitting time before and during COVID-19, and linear mixed models examined the association of outcome variables with PSS, time, and PSS × time. Results. Vigorous, moderate, and light PA decreased by 40%, 35% and 19%, sitting time increased by 52%, and PSS increase by 42% from before to during COVID-19. Conclusions: The perceived stress during COVID-19 may have influenced physical activity and sitting time. Effective health promotion programs and coping strategies are needed to protect college students as campuses reopen during COVID.