Only a small percentage of adults engage in regular physical activity, even though it is widely recommended as beneficial for well-being. Thus, it is essential to identify factors that can promote increased physical activity among adults of all ages. The current study examined the relationship of social media use to physical activity and emotional well-being. The sample is from the Midlife in the United States Refresher daily diary study, which includes 782 adults ages 25-75 years. Results showed that those who used social media less often engaged in more frequent physical activity, which, in turn, led to more positive affect. This relationship was found for midlife and older adults but not younger adults. The findings show the benefits of physical activity for well-being and suggest that social media use may dampen efforts to increase physical activity, especially among middle-aged and older adults.