Malnutrition is associated with prognosis in cancer. The geriatric nutritional risk index (GNRI), based on the ratio of actual to ideal body weight and also serum albumin level, is a simple screening tool for assessing nutrition. We investigated the GNRI as a prognostic factor for oncological outcomes in patients with high-risk metastatic hormone-sensitive prostate cancer (mHSPC) using a Japanese multicenter cohort. This study included a total of 175 patients with LATITUDE high-risk mHSPC, of whom 102 had received androgen deprivation therapy (ADT) plus upfront abiraterone acetate, and 73 had received ADT plus bicalutamide (Bica), from 14 institutions associated with the Tokai Urologic Oncology Research Seminar. Patients were classified into GNRI-low (<98) or GNRI-high (≥98) groups. The GNRI was based on the body mass index and serum albumin level. Kaplan-Meier analysis revealed that the median overall survival (OS) of a GNRI-low group (median 33.7 months; 95% confidence interval [CI]: 26.2-not reached [NR]) was significantly worse than that of a GNRI-high group (median: NR; 95% CI: NR-NR; < 0.001). Multivariate analysis identified Bica and low GNRI (<98) as independent prognostic factors for reduced times to both castration-resistant prostate cancer and OS, and, therefore, a poor prognosis. Our findings indicate the GNRI may be a practical prognostic indicator in the evaluation of survival outcomes in patients with LATITUDE high-risk mHSPC.