Background: Although definitive external-beam radiotherapy (EBRT) is one of the treatment options for non-metastatic castration-resistant prostate cancer (NM-CRPC), there are limited data on the long-term outcomes of this treatment. Methods: We retrospectively evaluated 31 NM-CRPC patients consecutively treated with definitive EBRT. The median age was 74 years upon EBRT initiation. The initial T stage distribution was as follows: T1c in 3, T2 in 11, T3 in 14, and T4 in 3 cases, respectively. The median prostate dose was 70.4 Gy. A castration-resistant status was defined as continuously increasing serum prostate-specific antigen levels despite ongoing hormonal therapy (HT). Results: The median follow-up duration after EBRT was 66.6 months. The median period of primary HT was 18.0 months. The 5- and 8-year overall survival rates were 74.6 and 49.8%, respectively. The 5- and 8-year prostate cancer-specific survival rates were 77.4 and 51.7%, respectively. Fourteen patients died, and prostate cancer was the cause of death in 12 of these patients. The 5- and 8-year relapse-free survival rates were 32.3 and 25.8%, respectively. Among 23 patients who experienced biochemical or clinical failure, the median duration to recurrence after EBRT was 19.3 months. The 5- and 8-year clinical failure-free survival rates were 56.0 and 51.4%, respectively. Among the 14 patients who experienced clinical failure, the median duration after EBRT was 16.0 months. The local relapse-free rates at 5 and 8 years were 91.0 and 91.0%, respectively. Grade 3 or higher adverse events were observed in four patients. Conclusion: Definitive EBRT achieved a long-term disease-free and clinical failure-free status in approximately one-third of and half of the treated NM-CRPC patients, respectively. This approach was also associated with favorable local relapse-free rates and overall survival outcomes. Definitive EBRT is a promising approach for NM-CRPC patients.