["Applied Psychology: Health and Well-Being, Volume 18, Issue 4, August 2026. ", "\nAbstract\nSedentary behavior poses significant risks to older adults yet remains an understudied health risk behavior. Studies typically overlook the importance of subject‐level variability in psychological constructs as predictors of sedentary behavior, instead focusing on the role that typical or mean levels of these constructs play in predicting behavior. This study uses ecological momentary assessment and device‐based sedentary behavior to determine the role of subject‐level variability in psychological constructs as predictors of sedentary behavior in older adults. Findings reveal that subject‐level variability in positive affect, but not motivational or self‐regulatory constructs, is associated with sedentary behavior (total sedentary behavior: β = −18.136, SE = 8.284, p = .029; bouted sedentary behavior: β = −16.534, SE = 7.814, p = .034). The association between subject‐level variability and total sedentary behavior tended to be positive but marginally significant among participants with lower mean levels of positive affect but was negative and significantly different from zero among participants with higher mean levels of positive affect. These findings highlight the importance of considering not only mean levels but also within‐person fluctuations in psychological constructs as key predictors of sedentary behavior. Studies of this kind can inform the development of personalized interventions, tailored to patterns of determinants in key constructs.\n"]