© 2022 The Authors. Nutrition in Clinical Practice published by Wiley Periodicals LLC on behalf of American Society for Parenteral and Enteral Nutrition.Background: The evaluation of body composition is an essential parameter for preventing obesity and sarcopenic obesity, which are prognostic factors in breast cancer. This study aims to validate the bioelectrical impedance analysis (BIA) of women who are breast cancer survivors by using the dual-energy x-ray absorptiometry (DXA) measurement method. Methods: This validation study included 104 women without metastasis between 32 and 72 years old (mean 47.03 ± 8.59) whose treatment was completed 6 months prior. Body composition analysis was performed sequentially using both measurements and when participants were hungry. Results: Meaningful differences were found in fat-free mass (FFM) (BIA: 46.57 ± 5.54 kg; DXA: 41.06 ± 5.11 kg), body fat percentage (%BF) (BIA: 34.28% ± 6.24%; DXA: 43.91% ± 5.58%), body fat mass (FM) (BIA: 25.37 ± 8.84 kg; DXA: 31.24 ± 9.09 kg), and lean soft tissue mass (LSTM) (BIA: 4.42 ± 5.66 kg; DXA: 38.75 ± 4.98 kg) (P < 0.001). Powerful associations for body FM and strong associations for other parameters were seen. A constant and/or proportional error was found between the two devices within the direction of strong and solid components. Compared with DXA, the BIA measurement gives a lower estimate of %BF and FM and a higher estimate of LSTM and FFM. Conclusions: By the mathematical relationship between the two measurement methods, it seems possible to adapt the body composition parameters obtained from BIA of patients with breast cancer to DXA results. In the future, there will be a need to evaluate these two devices with more extensive studies.