Background: There have been no prospective randomized controlled trials comparing current treatment options for patients with high-risk localized prostate cancer. This study seeks to compare the biochemical and metastatic outcomes of patients that received definitive radiotherapy (dRT) or radical prostatectomy (RP) for localized prostate cancer with Gleason score ≥ 8 on initial biopsy. Methods: A total of 106 patients met the inclusion criteria of Gleason score ≥ 8 on initial biopsy and biochemical follow-up ≥ 1 year. Seventy-one patients were initially treated with dRT (96% also receiving androgen deprivation therapy) and 35 patients were initially treated with RP (with or without postoperative RT). Our primary endpoint was biochemical failure (BF). For dRT patients, BF was recorded according to the Phoenix Consensus or if extranodal metastasis was diagnosed. For surgical patients, BF was recorded according to American Urological Association guidelines or if extranodal metastasis occurred. If adjuvant/salvage RT was given postoperatively, BF was recorded if PSA ≥ 0.5 on two consecutive measures after completion of RT. Pretreatment characteristics were compared using Pearson Chi-square method and independent samples Mann-Whitney U test. Actuarial rates of BF and metastasis were calculated using the Kaplan-Meier method. Results: Median follow-up for all patients was 5.3 years. There was no statistical difference in clinical T-stage, initial PSA, or months of follow up between patients treated initially with radiotherapy vs. prostatectomy. Patients initially treated with dRT were significantly older than those treated with RP. The dRT group had a lower rate of BF compared to the RP group, p < 0.001. The Kaplan-Meier estimate of BF at 5 years was 7.6% in the dRT group compared to 34.5% in the RP group. Additionally, the Kaplan-Meier estimate of distant metastasis at 10 years was 22.7% in the dRT group compared to 55.9% of the RP group, p = 0.01. Conclusions: For our sample of patients with Gleason score ≥ 8 on initial biopsy, initial treatment with dRT was associated with lower rates of biochemical failure and extranodal metastasis when compared to initial treatment with prostatectomy.