<b><i>Objectives:</i></b> The aim of the study was to examine the effectiveness of a modified-short hydration gemcitabine and cisplatin (m-shGC) regimen for patients with metastatic urothelial carcinoma (mUC) and to assess the efficacy of a geriatric nutritional risk index (GNRI) with regard to prognosis. <b><i>Patients and Methods:</i></b> From January 2016 to July 2020, 68 patients with mUC underwent first-line m-shGC therapy with 70 mg/m<sup>2</sup> cisplatin and 1,000 mg/m<sup>2</sup> gemcitabine (days 1, 8, and 15), with 2,050 mL fluid replaced on the first day of each 28-day cycle. Prior to the start of treatment, the serum neutrophil-to-lymphocyte ratio (NLR), and levels of albumin and C-reactive protein (CRP) in serum, as well as body heights and weights were measured. Patients were grouped according to GNRI <92 (low) or ≥92 (high). The analysis of data was done retrospectively. <b><i>Results:</i></b> Median follow-up was found to be 12.9 (range 1.7–50.2) months and the objective response rate (ORR) was 54.4% after m-shGC treatment. The ORR was significantly different when high and low-GNRI groups were compared (ORR: 28.0 vs. 69.8% in low- vs. high-GNRI groups). Median overall survival (OS) was calculated as 8.6 (95% confidence interval [CI]: 5.4–21.3) and 34.5 (95% CI: 20.5–NA) months for low- and high-GNRI groups, respectively (<i>p</i> < 0.0001). Unlike for NLR and CRP, univariate and multivariate analyses revealed that low GNRI and visceral metastases were significant prognostic factors for short OS. <b><i>Conclusions:</i></b> First-line m-shGC showed a survival benefit for mUC, with GNRI a useful prognostic biomarker.