Background: Short PSA doubling time (PSADT) after biochemical recurrence (BR) post radical prostatectomy (RP) is known to predict worse outcomes following salvage external beam radiation therapy (SRT). The ideal PSADT cut-off, however, in this context remains uncertain. In this study, we sought to identify the best PSADT cut-off for predicting clinical outcomes following SRT for BR after RP. Methods: 575 patients who received SRT at a single institution for BR after RP were retrospectively reviewed in an IRB approved analysis. The impact of PSADT on biochemical failure (BF), distant metastasis (DM), prostate cancer-specific mortality (PCSM), and overall survival (OS) was assessed using Kaplan-Meier and Cox Proportional Hazards models. Results: Median follow up was 56.7 months post SRT. PSADTs could be calculated for 277 patients. PSADT strongly predicted BF, DM, PCSM, and OS on univariate analysis regardless of cut-off point. There was no statistical difference in BF, DM, PCSM, or OS between patients with PSADT <3 (n=40) and 3-6 months (n=61) or between 6-10 (n=62) and >10 months (n=114). A difference existed in BF (p<0.01 HR: 2.2 [95%CI: 1.4-3.5]) and DM (p=0.02 HR: 2.2 [95%CI: 1.2-4.3]) between PSADT of 3-6 and 6-10 months. PSADT ≤6 had the largest positive predictive value (PPV) for BF (70%), DM (36%), and PCSM (13%) at 5 years. There was no difference in negative predictive value between a PSADT >10 vs. >6 months for BF, DM, PCSM, and OS with 5 year rates of (60% vs. 60%, 86% vs. 86%, 99% vs. 98%, and 95 vs. 94% respectively). On multivariate analysis PSADT ≤6 was a strong predictor of BF (p<0.01 HR: 2.1 [95%CI: 1.5-3.0]), DM (p=0.01 HR: 2.0 [95%CI: 1.2-3.4]), and PCSM (p=0.04 HR: 2.3 [95%CI: 1.1-5.2]), with a trend towards predicting OS (p=0.12 HR: 1.5 [95%CI: 0.9-2.6]). Conclusions: A PSADT ≤6 months was the best predictor of outcomes in our data set, particularly for DM and PCSM. Currently, the most common predictive nomogram for SRT uses PSADT <10 months as the cut-off point for BF. These results suggest that using a PSADT of ≤6 months may improve the ability to predict clinically significant outcomes and hence identify men who may benefit from additional therapy.