<p>Recent advancements in imaging have improved the potential of computed tomography (CT) for body composition assessment. However, dual-energy X-ray absorptiometry (DXA) and CT are not clearly interchangeable for assessing appendicular skeletal muscle mass (ASM). This study aimed to predict ASM using a single abdominal CT slice in patients who underwent gastrectomy. This retrospective study included patients scheduled for or who underwent gastrectomy with an interval of 0–1&#xa0;day between DXA and abdominal CT scans. Data on six variables were extracted: age, sex, weight, height, muscle area, and muscle density on abdominal CT. CT parameters were measured at the 3rd lumbar (L3) and upper thigh levels, respectively. A total of 221 datasets from 112 patients were included. The mean age was 62.0 years (range, 30–89 years) and the average body mass index was 21.3 ± 3.1&#xa0;kg/m². More patients were male (58.8%), and 204 (92.3%) examinations were performed after gastrectomy. The variables that significantly affected the ASM were muscle area, weight, height, and sex. The predictive model for ASM revealed high R² values (0.915 and 0.918 for the L3 and upper thigh levels, respectively). The predictive model for ASM of DXA with clinical and CT parameters showed high performance, accordingly, CT scans can be used as a surrogate maker for ASM.</p>