Daily fluctuation in pain acceptance and its impact on the physical and psychosocial functioning of individuals living with spinal cord injury (SCI) and chronic pain has not been examined. We used end-of-day diaries and multilevel modeling to examine the moderating effect of within- and between-person pain acceptance on associations between pain and physical and psychosocial functioning. Individuals with SCI and chronic pain (N = 124) completed 7 days of end-of-day diaries, which included measures of pain acceptance, pain intensity, pain catastrophizing, pain interference, participation in social roles and activities, depressive symptoms, and positive affect and well-being. We found within-person variability in pain acceptance (28% of the total variance) and a significant moderating effect of daily fluctuation in pain acceptance on the same-day pain intensity-social roles and activities association. Within-person changes in pain acceptance were also associated with daily changes in pain interference, depressive symptoms, and positive affect and well-being, adjusting for pain intensity and catastrophizing. Findings highlight the potential for daily or momentary assessments of pain acceptance to enhance understanding of how psychological flexibility may contribute to pain-related outcomes. Future studies could further investigate stable and variable characteristics of pain acceptance and their individual contribution to physical and psychosocial functioning.