<b><i>Background:</i></b> Participation in activities is associated with a range of positive outcomes in adulthood. Research has shown that pain and physical symptoms are associated with less activity in older adults, whereas higher self-efficacy is associated with more activity. Such research tends to examine cross-sectional or long-term between-person change, limiting the opportunity to explore dynamic within-person processes that unfold over shorter time periods. <b><i>Objectives:</i></b> This study aimed to (1) replicate previous between-person associations of self-efficacy with engagement in activity and (2) examine whether daily variation in pain, physical symptoms, and self-efficacy corresponded with daily within-person variation in different types of activity. We predicted that participants would engage in less activity on days when they experienced more pain or physical symptoms than their average (a negative within-person association) and that participants would engage in more activity on days when self-efficacy was higher than average (a positive within-person association). <b><i>Methods:</i></b> This study used an online diary study to assess self- reported daily pain, physical symptoms, self-efficacy, and engagement in activity among 185 adults aged 51-84 years for up to 7 days. Multilevel modelling was used to examine whether between-person (average) and daily within-person variability in pain, physical symptoms, and self-efficacy were associated with social, physical, and mental activity. <b><i>Results:</i></b> In line with previous research, between-person self-efficacy was positively associated with social and physical activity. Supporting the hypotheses, within-person self-efficacy was also positively associated with social and physical activity. The results for pain and physical symptoms were less consistent. Between-person pain was positively associated with social activity. Age interactions indicated that within-person pain was negatively associated with social activity and positively associated with physical activity among older adults. Within-person physical symptoms were positively related to social and mental activity. <b><i>Conclusion:</i></b> Stable individual differences as well as short-term within-person variation in physical and psychological functioning are associated with day-to-day variation in activity. Between-person associations did not always reflect within-person associations (e.g., for pain). These complex associations may be influenced by a range of factors including the type of activity and how it is defined (e.g., specific activities and their difficulty), the type of physical symptoms experienced, and age.