Cancer cachexia is associated with poor prognosis in patients with metastatic urothelial carcinoma (mUC). The objective of the study was to assess the cachexia index (CXI), which is a new indicator assessing the status of cancer cachexia, as a prognostic indicator for mUC patients treated with gemcitabine plus cisplatin (GC) chemotherapy. The study included 55 patients with mUC who underwent GC chemotherapy between 2008 and 2022 as first-line chemotherapy. The CXI at the start of chemotherapy was determined as follows: CXI=(serum albumin × skeletal muscle mass index)/ (neutrophil count/lymphocyte count). Patients were categorized into two groups based on a median CXI value (CXI-high and CXI low). We used Kaplan-Meier curves and multivariate Cox proportional hazards regression models to assess the association between the CXI and overall survival (OS). At the start of GC chemotherapy, significant differences were not found in patients' characteristics. The median OS was significantly shorter in the CXI-low group [10.0 months (95% confidence interval (CI)=5.1-12.8)] than in the CXI-high group [22.3 months (95% CI=13.6-NA), p<0.05]. Multivariate analysis revealed that low CXI was a predictor of a poor prognosis [hazard ratio (HR)=2.25, 95% CI=1.12-4.52, p<0.05]. CXI might be useful as a prognostic indicator for patients with mUC undergoing first-line GC chemotherapy.