Clinically elevated rates of post-traumatic stress disorder (PTSD) symptoms are found among many youth with chronic pain and their parents and are linked to worse child pain outcomes. Conceptual models of mutual maintenance in pediatric PTSD and chronic pain posit that child and parent pain catastrophizing are key mechanisms underlying this co-occurrence. To our knowledge, the current study is the first to examine child and parent pain catastrophizing as potential mediators in the child PTSD-child pain and parent PTSD-child pain relationships among a cohort of youth with chronic pain. One hundred two children (72.5% female, mean age = 13.5 years), recruited from a tertiary level chronic pain program, and 1 of their parents participated. At intake, parents completed psychometrically sound self-report measures of PTSD symptoms and catastrophizing about child pain. Children completed self-report measures of PTSD symptoms, pain catastrophizing, pain interference, and pain intensity. Findings revealed that relationships between child PTSD and child pain as well as parent PTSD and child pain were mediated by child (but not parent) pain catastrophizing. This suggests that children's catastrophic thinking about pain may explain how child and parent PTSD symptoms influence children's experience of chronic pain and is a potential target in family-based interventions to improve pain and mental health outcomes. PerspectiveConsistent with conceptual models of co-occurring PTSD and chronic pain, children's catastrophic thinking about child pain mediated relationships between parent and child PTSD symptoms and child chronic pain outcomes. Child pain catastrophizing may be a fruitful target in interventions to improve children's chronic pain and mental health outcomes.