Background: Childhood cancer affects and is affected by multiple levels of the social ecology, including social and relational determinants of health (e.g. economic stability, housing, childcare, healthcare access, child and family problems). The 2015 Standards of Psychosocial Care in Pediatric Cancer outline optimal psychosocial care sensitive to these ecological factors, starting with assessment of psychosocial healthcare needs to promote medical and psychosocial outcomes across all children with cancer. To address the first standard of family psychosocial assessment, the Psychosocial Assessment Tool (PAT) is a validated screener ready for broad implementation. Method: The PAT will be implemented across a national sample of 18 pediatric cancer programs ranging in size (annual new patients) in a mixed methods, comparative effectiveness study, guided by the Interactive Systems Framework for Dissemination and Implementation, comparing two implementation Strategies. It is hypothesized that implementation will be more successful at the patient/family, provider, and institutional level when Training (Strategy I) is combined with Implementation Expanded Resources (Strategy II). There are three aims: 1) Refine the two implementation strategies using semi-structured qualitative interviews with 19 stakeholders including parent advocates, providers, pediatric oncology organization representatives, healthcare industry leaders; 2) Compare the two theoretically based and empirically informed strategies to implement the PAT in English and Spanish using a cluster randomized controlled trial across 18 sites. Stratified by size, sites will be randomized to cohort (3) and strategy (2). Outcomes include adoption and penetration of screening (patient/family), staff job satisfaction/burnout (provider), and cost effective use of resources consistent with family risk (institution); 3) Based on the results of the trial and feedback from the first and second aim, we will develop and disseminate a web-based PAT Implementation Toolkit. Discussion: Use of the PAT across children’s cancer programs nationally can achieve the assessment Standard and inform equitable delivery of psychosocial care matched to family need for all patients.Trial Registration: ClinicalTrials.gov, NCT04446728, registered 23 June 2020