Simple Summary Gastric cancer (GC) is one of the most common cancers and fourth for mortality of all malignancies globally. Patients with GC had a high prevalence of cachexia. However, the current diagnostic criteria for cancer cachexia are inconsistent, and the prognostic value of cachexia in GC is controversial. In this study, we investigated the prognostic value of the cachexia index (CXI), a new measurement of cachexia, in 324 patients with GC. We demonstrated that low CXI could be a useful measurement of cachexia, which was verified by analyzing the associations between the CXI and TNM stage, serum nutritional and inflammatory markers, postoperative complications, and overall survival. We also found that the combination of CXI with cachexia, BMI, or TNM stage can more accurately distinguish patients with poor prognoses, which could be helpful to manage and support these patients early. The current diagnostic criteria for cancer cachexia are inconsistent, and the prognostic value of cachexia in gastric cancer (GC) is controversial. This study aimed to investigate the prognostic value of the cachexia index (CXI) in patients with GC. We calculated the CXI as skeletal muscle index (SMI) x serum albumin/neutrophil-lymphocyte ratio (NLR), and a total of 161 and 163 patients were included in the high and low CXI groups, respectively. Low CXI was significantly associated with a more advanced tumor-node-metastasis (TNM) stage, a higher level of serum C-reactive protein, serum interleukin-6, and NLR, but also a decreased level of serum prealbumin and albumin. In addition, patients in the low CXI group were more likely to have postoperative pulmonary infections (9.8% vs. 3.7%, p = 0.03). Cox proportional analyses indicated that patients with low CXI (HR 0.45, 95% CI 0.29 to 0.69; p < 0.001) or TNM stage III+IV (HR 4.38, 95% CI 2.54 to 7.55; p < 0.001) had a significantly poorer overall survival (OS). Kaplan-Meier survival curves suggested that patients with low CXI had a significantly decreased OS, which was not affected by subgroup analyses of different sex, age, cachexia, body mass index (BMI), and TNM stage. Furthermore, low CXI combined with cachexia, low BMI, or TNM stage III+IV caused the worst OS in each subgroup analysis, respectively. Our study demonstrated that CXI had a good prognostic value in GC. Greater attention should be paid to patients with low CXI, particularly those combined with cachexia, low BMI, or TNM stage III+IV.