Background: Body composition analysis is crucial in clinical practice, yet few methods are suitable for pediatric patients, and data on young children with obesity are limited. Methods: This study measured body fat percentage (BFP), fat mass (FM), and fat-free mass (FFM) in 26 pediatric patients with obesity (BMI: 35.6±6.9 kg/m²), using two bioelectrical impedance analysis (BIA) devices (TANITA and BIACORPUS), and the results were compared to those of the gold-standard dual-energy X-ray absorptiometry (DXA). Additionally, air displacement plethysmography (BODPOD) was compared with DXA, and all methods were evaluated against each other. Results: Significant differences were observed between all methods and parameters (p < 0.05). For example, Bland–Altman analysis for BFP identified differences between BIACORPUS and DXA (mean: −3.5%; 95% limits of agreement: −16.7% to 9.8%) and between TANITA and DXA (mean: −3.1%; 95% limits of agreement: −12.2% to 6.1%). These differences can be regarded as clinically relevant, especially when considering the 95% limits of agreement. Further, moderate differences between BODPOD and DXA were identified, which could be clinically relevant (mean: 2.1%; 95% limits of agreement: −4.2% to 8.5%). Conclusions: TANITA was the most comparable BIA method to the gold standard, DXA. Therefore, TANITA is recommended for assessing body composition in young patients with obesity to monitor therapy progress in clinical settings. When using BODPOD as an alternative to DXA, caution is warranted since we found relevant differences between both methods.

(1) Background: The determination of body composition is an important method to investigate patients with obesity and to evaluate the efficacy of individualized medical interventions. Bioelectrical impedance-based methods are non-invasive and widely applied but need to be validated for their use in young patients with obesity. (2) Methods: We compiled data from three independent studies on children and adolescents with obesity, measuring body composition with two bioelectrical impedance-based devices (TANITA and BIACORPUS). For a small patient group, additional data were collected with air displacement plethysmography (BOD POD) and dual-energy X-ray absorptiometry (DXA). (3) Results: Our combined data on 123 patients (age: 6–18 years, body mass index (BMI): 21–59 kg/m²) and the individual studies showed that TANITA and BIACORPUS yield significantly different results on body composition, TANITA overestimating body fat percentage and fat mass relative to BIACORPUS and underestimating fat-free mass (p < 0.001 for all three parameters). A Bland–Altman plot indicated little agreement between methods, which produce clinically relevant differences for all three parameters. We detected gender-specific differences with both methods, with body fat percentage being lower (p < 0.01) and fat-free mass higher (p < 0.001) in males than females. (4) Conclusions: Both bioelectrical impedance-based methods provide significantly different results on body composition in young patients with obesity and thus cannot be used interchangeably, requiring adherence to a specific device for repetitive measurements to ascertain comparability of data.