Objectives: We prospectively investigated risk factors for acute urinary retention (AUR) following transperineal radioactive seed implantation for prostate cancer. Methods: A total of 273 consecutive patients underwent transperineal ultrasound-guided prostate brachytherapy for clinical T1c–T3b prostate cancer. Preoperative factors included age; International Prostate Symptom score; planimetric prostate and transition volumes (TZV) measured by transrectal ultrasound; peak flow rate; post-void residual urine; neoadjuvant hormone therapy; use of pelvic radiation; and T stage. Intra- and postoperative factors included the number of seeds and needles. Results: Ten patients (3.6%) showed AUR requiring catheterization. Among preoperative factors, age, International Prostate Symptom score, planimetric prostate volume, planimetric prostate volume and radiation therapy (RT) were not significantly correlated with AUR. PFR (P = 0.012) and PVR (P = 0.020) of patients having AUR were significantly higher than those not having AUR. Mean TZV was also significantly higher (P = 0.038) in the AUR group univariate analysis. Multivariate analysis showed that PFR is the only independent predictor of AUR (P = 0.030). Conclusion: On univariate analysis, PFR, PVR and TZV were found to be statistically significant predictors of AUR following seed implantation. According to multivariate analysis, only PFR was confirmed to be a significant predictor.