The Knee injury and Osteoarthritis Outcome Score (KOOS) is a self-administered valid and reliable questionnaire for adults with joint injury or degenerative disease. Recent data indicate a lack of comprehensibility when this is used with children. Thus, a preliminary KOOS-Child was developed. This study aims to evaluate psychometric properties of the final KOOS-Child when used in children with knee disorders.115 children (boys/girls 51/64, 7-16 years) with knee disorders were recruited. All children (n=115) completed the KOOS-Child, the Child-Health Assessment Questionnaire (CHAQ) and the EQ-5D-Youth version (EQ-5D-Y) at baseline to evaluate construct validity. Two additional administrations (1-3 weeks and 3 months) were performed for analyses of reliability (internal consistency and test-retest; n=72) and responsiveness (n=91). An anchor-based approach was used to evaluate responsiveness and interpretability.After item reduction, the final KOOS-Child consists of 39 items divided into five subscales. No floor or ceiling effects (≤15%) were found. An exploratory factor analysis on subscale level demonstrated that items in all subscales except for Symptoms loaded on one factor (Eigenvalues 3.1-5.5, Symptom: 2 factors, Eigenvalue >1). Sufficient homogeneity was found for all subscales (Cronbach's α = 0.80-0.90) except for the Symptoms subscale (α = 0.59). Test-retest reliability was substantial to excellent for all subscales (Intraclass Correlation Coefficient 0.78-0.91, Smallest Detectable Change (SDC)ind 14.6-22.6, SDCgroup 1.7-2.7). Construct validity was confirmed, and greater effect sizes were seen in those reporting improved clinical status. Minimal important changes greater than the SDCs were found for patients reporting to be better and much better.The final KOOS-Child demonstrates good psychometric properties and supports the use of the KOOS-Child when evaluating children with knee disorders.