Background: Childhood obesity is one of the most critical global health challenges. Poor knee extensor strength is associated with disability and difficulties with physical function in obese patients. The objective of this study was to evaluate the test-retest reliability of peak torque and total work in concentric flexion and concentric extension in obese children. Methods: 25 obese children aged between 6 and 11 years performed 3F maximal unilateral isokinetic repetitions with the knee extensors/flexors performing concentric actions at 60°/s. Peak torque (Nm), total work (J), and the ratio antagonist/agonist were recorded and normalized by total and lean mass. The intraclass correlation coefficient (ICC), standard error of measurement (SEM), and smallest real difference (SRD) were computed. Results: Reliability was excellent for almost all variables. The highest ICC values were observed when work or peak torque were not divided by any other variable. However, SEM (%) and SRD (%) were lower when peak torque and work were divided by the total mass or lean mass. The antagonist/agonist ratio showed an ICC value of 0.873. Conclusion: Peak torque, total work, and the antagonist/agonist ratio are reliable in obese children.