Prosociality (i.e., voluntary thoughts and actions intended to benefit somebody else) is arguably essential for positive social relationships. The present research investigated under what conditions a prosocial focus has positive effects on the subjective well-being of a prosocial person. We addressed this question in an intervention study of daily social interactions. All participants (N = 295, 57.6% women, age 19-88 years) started the study with a baseline day with no intervention. Participants then underwent a video-based intervention that trained them to focus on either the well-being of others or the well-being of themselves. Participants applied the corresponding focus in social interactions on the next day. Compared with baseline, focusing on others did not significantly affect subjective well-being, whereas focusing on self significantly reduced subjective well-being (the latter effect was further moderated by age and relationship closeness). The difference between self- and other focus was moderated by age: Younger participants (<36 years) reported lower subjective well-being in the self-focus condition, whereas older adults (>68 years) showed the opposite effect (there was no difference in the middle-aged adults). Taken together, the results suggest that focusing on a social partner increases concerns about the partner's well-being but not necessarily one's own well-being. In contrast, focusing on oneself reduces concerns about the partner's well-being and-particularly for young adults-one's own well-being, rendering a strong focus on oneself a potential risk factor for young adults' well-being. Further research is needed to understand the negative effect of other- versus self-focus on older adults' well-being. (PsycInfo Database Record (c) 2021 APA, all rights reserved).