Prosocial activities, such as volunteering, predict better mental and physical health in late adulthood, but their proximal links to well-being in daily life are largely unknown. The current study examined day-to-day associations of prosocial activities with emotional and physical well-being, and whether these associations differ with age. We used daily diary data from the National Study of Daily Experiences (NSDE) II (n = 2,016; ages 33-84) and NSDE Refresher Study (n = 774; ages 25-75). Participants completed telephone interviews on 8 consecutive evenings regarding their prosocial activities (formal volunteering, providing unpaid assistance, providing emotional support), well-being (negative affect, stressors, positive events), and physical symptoms. On days when individuals participated in more formal volunteering or provided more unpaid assistance than usual, they experienced more stressors and positive events but no difference in the number of physical symptoms. Negative affect was reduced on volunteering days for older adults but increased for younger adults (NSDE Refresher). Providing emotional support was associated with higher same-day negative affect, more stressors, more positive events, and elevated physical symptoms. Compared to younger and middle-aged adults, older adults experienced less of an increase in stressors and positive events (NSDE II) and negative affect (NSDE Refresher) on days when they provided more emotional support than usual. These findings demonstrate that prosocial activities are associated with both costs (negative affect, stressors, physical symptoms) and benefits (positive events) for same-day well-being. Older age may protect against negative ramifications associated with prosocial activities. (PsycInfo Database Record (c) 2021 APA, all rights reserved).