Pediatric and adolescent and young adult (AYA) thromboembolism is treated with anticoagulation. While anticoagulation non-adherence in adults has been linked to increased morbidity and mortality, much is unknown concerning medication adherence in pediatric and AYA populations. The aims of this study were to describe barriers to adherence in anticoagulated pediatric and AYA outpatients and to explore the relationship between barriers and self-reported adherence. Nearly 75% of patients and caregivers reported barriers to anticoagulation adherence, and a greater number of reported barriers was associated with lower adherence (rpb = 0.48, p = .01). Limitations, clinical implications, and future directions are discussed.