Introduction: Cancer cachexia occurs in cancer patients more frequently as the cancer progresses, with a negative impact on treatment outcomes. In this study, we sought to clarify the clinical impact of a cancer cachexia index (CXI) in patients with gastric cancer (GC) undergoing gastrectomy. Methods: Between January 2013 and December 2018, we reviewed data from 556 patients treated for GC at our hospital. CXI was calculated using skeletal muscle index (SMI), serum albumin, and neutrophil-lymphocyte ratios (NLR). Patients were divided into high (n = 414) or low CXI (n = 142) groups. We investigated the clinical impact of CXI in patients with GC undergoing gastrectomy. Results: Multivariate analyses of 5-year overall survival (OS) and cancer-specific survival (CSS) rates indicated that a low CXI was independently associated with unfavorable outcomes for patients with GC. In multivariate analyses, SMI was independent predictor of OS but not CSS. NLR was not an independent predictor of either OS or CSS. Complication incidences (≥ Clavien Dindo 3) were non-significantly higher in the low (vs. high) CXI group. Conclusion: CXI was a more valuable prognostic biomarker when compared with SMI or NLR in GC patients undergoing gastrectomy. We suggest that patients with low CXI values should be given more comprehensive treatment, including exercise and nutritional therapy to improve clinical outcomes.