Purpose: Evidence on the use of definitive radiotherapy (RT) in younger prostate cancer (PCa) patients is limited, with no national-level studies systematically evaluating oncologic outcomes and treatment-related toxicity in this population. This study aimed to assess oncologic outcomes and toxicity of definitive prostate RT in patients aged 55 years or younger. Methods: Records of 180 PCa patients aged ≤ 55 years treated with definitive RT ± androgen deprivation therapy across 12 cancer centers were retrospectively reviewed. Primary endpoints included freedom from biochemical failure (FFBF) and PCa-specific survival (PCSS). Secondary endpoints were overall survival (OS), acute/late genitourinary (GU) and gastrointestinal (GI) toxicities, local recurrence (LR), and distant metastasis (DM). Results: Median age was 54 years; median PSA at diagnosis was 10.4 ng/mL. Risk group distribution was 29% low-, 29% intermediate-, and 42% high-risk. Median RT dose was 76 Gy delivered in 38 fractions. After a median follow-up of 106.2 months, local recurrence (9%), distant metastasis (7%), and isolated PSA progression (3%) were noted. The 8‑year FFBF, PCSS, and OS were 85.7%, 93.8%, and 90.3%, respectively. Univariate analysis identified PSA, clinical T stage, Gleason score (GS), and risk group as significant prognostic factors for FFBF and PCSS. Multivariate analysis showed advanced stage and high GS independently predicted worse FFBF and PCSS. Acute and late grade ≥ 2 GU toxicity occurred in 12% and 6%, and GI toxicity in 11% and 3%. Conclusion: Definitive radiotherapy is a safe and effective treatment for prostate cancer patients aged ≤ 55 years. Prospective studies are needed to confirm these findings.