Background: The wellbeing of Children Looked After (CLA) is a multifaceted construct given their adverse experiences and greater risk of mental health complications. There is a clear need for a comprehensive mental health and psycho-adjustment measure for diagnostic and monitoring purposes within this population. Objective: The aim of this study is to test, explore and validate the optimal factor structure of a new mental health and psychoadjustment measure looking at Behavior, Emotions, Relationship, Risk, and Indicators (BERRI). Participants and Setting: The data were extracted from an existing large naturalistic sample of CLA for whom the BERRI was used as part of their placement providers' service provision (N = 1543, 45% female). The surveys were completed by their primary carer reporting the child's experiences during the previous month.Methods: Both an exploratory factor analysis (EFA) and a confirmatory factor analysis (CFA) were conducted to assess the 87-item measure's initial factor structure. For cross-validation purposes, the database was split 60-40 between a 'train' and a 'test' sub-sample.Results: An eight-factor structure (Dysregulation, Vulnerability, Neurodiverse needs, Negative self and emotions, Emotional disconnect, Barriers to connection, Risk to self, and Disrupted routines) emerged. This was subsequently validated by the CFA suggesting a good fit (CFI = 0.951, SRMR = 0.08, and RMSEA = 0.038) and good internal consistency.Conclusions: Our study supported the structure validity and internal consistency of a mental health and psychoadjustment measure specifically developed for CLA. Recommendations for further research relating to the tool's applicability in other populations are proposed.