Background: Altered body composition is common in patients undergoing major hepatopancreaticobiliary (HPB) surgery and may be associated with increased postoperative morbidity. Conventional markers such as body mass index (BMI) and serum albumin have limited sensitivity for detecting early nutritional changes. Bioelectrical impedance analysis (BIA) provides an objective assessment of body composition, and phase angle (PA) may serve as a proxy for cellular status. Methods: This single-center retrospective observational cohort study included 73 patients undergoing major HPB surgery. Preoperative BIA-derived variables included resistance (R), reactance (Xc), PA, fat-free mass index (FFMI), fat mass index (FMI), and the Body Composition Chart (BCC). Nutritional Risk Screening 2002 (NRS 2002), serum albumin, and prealbumin were assessed preoperatively. BIA-derived data were compared with published healthy reference data. Postoperative complications were graded according to the Clavien–Dindo classification. Results: Patients demonstrated significant alterations in BIA-derived variables compared with healthy controls. Xc and PA were significantly reduced across all analyzable/matched sex and BMI strata (all p < 0.05). The median PA in the surgical cohort was 4.3° and significantly lower than reference values for both female and male patients (p < 0.001). FFMI and FMI distributions indicated a high prevalence of adverse body composition phenotypes, including low fat-free mass and increased adiposity. In multivariable Firth penalized logistic regression, a BCC classification outside the green reference zone (odds ratio (OR) 3.32, 95% confidence interval (CI) 1.04–12.87, p = 0.043) and higher Eastern Cooperative Oncology Group (ECOG) performance status (OR 2.07, 95% CI 1.08–4.24, p = 0.028) were independently associated with severe postoperative complications. Conclusions: Patients undergoing major HPB surgery frequently present with relevant preoperative nutritional and cellular impairments that are not adequately identified by BMI and serum albumin. Incorporation of BIA into routine preoperative assessment may improve risk stratification and support targeted nutritional optimization in this high-risk surgical population.