["Japanese Psychological Research, EarlyView. ", "\nAbstract\nThe beneficial role of social relationships in the well‐being of older adults has been well documented. However, little is known about how social interactions contribute to well‐being in very old adults aged 80 years and above. This study examined between‐ and within‐person associations between social interactions and affect in very old adults' daily lives. Fifty‐seven community dwellers (age 82 to 86 years) responded to a daily diary questionnaire regarding whether they had direct and indirect social interactions with others and how frequently they felt positive and negative affect in the evenings over 7 days. At the between‐person level, older adults who directly interacted with others more frequently reported higher levels of positive affect than those who did less. At the within‐person level, on days with direct social interactions, older adults reported higher levels of positive affect than on days without such interactions. The within‐person association between direct social interactions and positive affect was more evident in older adults living alone than in those co‐residing with others. The findings suggest that daily face‐to‐face social interactions may enhance the daily well‐being of very old adults, particularly those with limited social relationships.\n"]