This 14-day daily diary study tested the between-person and within-person associations between sedentary behavior, physical activity, and life satisfaction in community-dwelling older adults. Older adults (n = 100) wore ActivPAL3 activity monitors for 14 days and, at the end of each day, answered questions regarding their health behaviors and life satisfaction. Separate multilevel models were tested for self-reported and objectively measured behavioral data. In the model using objectively measured behavioral data, life satisfaction was (a) negatively associated with sedentary behavior at the within-person level and unassociated with sedentary behavior at the between-person level and (b) unassociated with physical activity at either the between-person or within-person level. In the model using self-reported behavioral data, life satisfaction was (a) unassociated with sedentary behavior at either the between-person or within-person level and (2) positively associated with physical activity at the within-person, but not at the between-person, level. Results indicated that daily deviations in objectively measured sedentary behavior and self-reported physical activity have implications for older adults' well-being. Interventions designed to enhance well-being and quality of life in older adults should consider targeting daily changes in total sedentary behavior and daily changes in the volume or frequency of physical activity.