Background: Research among adult and paediatric samples suggests that pain-related injustice appraisals contribute to adverse pain-related functioning. However, a singular focus on pain-related injustice appraisals carries the risk of underestimating the role of broader concepts of justice. This study examined the unique roles of child pain-related injustice appraisals and just-world beliefs in understanding disability and physical, emotional, social and academic functioning, as well as the mediating role of injustice appraisals in the relationship between just-world beliefs and functioning. Methods: Participants comprised a school sample of 2,174 children (Study 1) and a clinical sample of 146 paediatric chronic pain patients (Study 2) who completed the Injustice Experience Questionnaire (IEQ), Personal and General Belief in a Just World scales (JWB-P/G), Functional Disability Inventory (FDI), Pain Catastrophizing Scale for Children (PCS-C) and Pediatric Quality of Life Inventory (PEDSQL). Results: For both samples, child pain-related injustice appraisals were associated with poorer functioning, after controlling for just-world beliefs, catastrophizing, pain intensity, age and sex. In the school sample, injustice appraisals mediated the associations of both personal and general just-world beliefs with functioning. In the clinical sample, injustice appraisals mediated the association of personal, but not general, just-world beliefs with all functioning scales. Conclusions: The current findings attest to the unique role of pain-related injustice appraisals in understanding child pain-related functioning and their explanatory value in understanding the relationship between fundamental just-world beliefs and child pain-related functioning. Significance: The present study adds to emerging literature on the adverse effects of child pain-related injustice appraisals in the context of pain, through showing that pain-related injustice appraisals are uniquely associated with pain-related functioning and mediate the relationship between just-world beliefs and pain-related functioning. These findings suggest that interventions may target pain-related injustice appraisals as a mechanism for change in children.