Pediatric traumatic brain injury (TBI) can impact psycho-social functioning, including friendships and experiences of loneliness; however, few studies have explored associations between these factors and self-esteem. To address this gap, the objective of this study was to document inter-relationships between friendship quality, peer-related loneliness, and self-esteem 12 months post-injury. The sample in this prospective, longitudinal observational study consisted of 135 children, including 92 children with TBI (57 mild, 35 moderate-severe TBI) and 43 typically developing control (TDC) children. Children rated their friendship quality, loneliness, and self-esteem. Parents completed questionnaires measuring socioeconomic status and children's pre-injury functioning. At 12-month follow-up, ratings of friendship quality (perceived support and satisfaction with friendships), peer-related loneliness, and self-esteem were comparable between TBI and TDC participants. In mediation models, poorer quality friendships were associated with greater peer-related loneliness, and greater peer-related loneliness was related to reduced athletic and social self-esteem, at 12 months post-TBI. Loneliness with peers mediated the effects of both friendship support and friendship satisfaction on children's social self-esteem. Our preliminary findings suggest that regular monitoring of social interactions post-TBI might facilitate early identification of children who are likely to benefit from interventions targeting peer-related loneliness, which in turn may lower risk for poorer self-esteem.