Posttraumatic stress disorder (PTSD) symptoms are associated with increased physical health problems, including chronic pain. Although researchers have identified several risk factors that partially account for the co-occurrence of PTSD symptoms and pain, a substantial amount of variance remains unexplained in these models. The present study evaluated psychological flexibility (PF) as a potential moderator of the relation between probable PTSD diagnosis and pain interference in daily life. Probable PTSD participants reported significantly greater daily pain severity and interference, and greater daily pain interference when controlling for pain severity, relative to non-PTSD participants. PF emerged as a significant moderator of the relation of PTSD and pain interference. Participants with probable PTSD reported greater pain interference only in conjunction with low levels of PF. Results suggest psychosocial treatments that target PF as a means of improving functioning may improve outcomes for patients with co-occurring PTSD and chronic pain.