Musculoskeletal disorders such as knee osteoarthritis (OA) are the primary cause of chronic pain in older adults. Recommended self-management strategies for knee OA include staying physically active in the face of pain, but many patients avoid activities they are capable of doing. The overall purpose of this study was to examine the extent to which daily pain catastrophizing, a maladaptive coping strategy, could influence OA patients' physical activity and sedentary behavior. The current study used data from 143 older knee OA patients who completed electronic daily diaries for 22 days and wore an accelerometer to capture physical activity and sedentary behavior. At the beginning of each day, patients reported their pain catastrophizing regarding the day ahead. Results from multilevel models demonstrated that on mornings when patients catastrophized more than usual about their pain in the day ahead, they spent more time in sedentary behavior and engaged in fewer minutes of moderate to vigorous physical activity that day. Cross-day lagged analyses further showed that the effect of morning pain catastrophizing on subsequent sedentary behavior extended to the next day. More time spent in sedentary behavior, in turn, contributed to greater pain catastrophizing the next morning. These findings support the mechanistic role of daily pain catastrophizing in the avoidance of physical activity for older OA patients, and suggest that effective interventions for pain catastrophizing may also reduce sedentary behavior and enhance physical activity, with longer-term benefits for pain management, physical function, and overall health.