Purpose: To report on rectal dosimetry and toxicity outcomes in men with prostate cancer (PCa) treated with iodine-125 low-dose-rate brachytherapy (LDR-BT) with or without polyethylene glycol hydrogel (HS) or hyaluronic acid (HA) rectal spacer (RS) insertion.Material and methods: Seventy consecutive men treated with LDR-BT between December 2017 and July 2019 were included in this study, including twenty-eight (40%) men who had RS insertion according to the preference of referring urologist, compared to a group of forty-two men (60%) without RS. Descriptive statistics were used to compare RS safety, dosimetric effects on organs at risk (rectum and urethra), and gastrointestinal (GI) and genitourinary toxicities (GU) (assessed using the CTCAE v.4) between the two groups of patients.Results: The median prostate-rectal separation with RS at mid prostate was 10 mm (IQR, 8-11.5 mm). The median follow-up was 23.5 months. There were no post-operative complications for RS insertion. There was significantly reduced rectal dosimetry in RS-group vs. non-RS group; the median RV 100 was 0.0 cc (IQR, 0.0-0.0 cc) vs. 0.4 cc (IQR, 0.1-1.1 cc) (p < 0.001), respectively. The mean rectal D 1cc and D 2cc were 52.4% vs. 84.2% (p < 0.001) and 45.7% vs. 70.0% (p < 0.001) for RS and non-RS group, respectively. There were no statistically significant differences in the mean urethral D 20 , D 5 , and D 1 . There were significantly less grade 1 acute and late GI toxicities in RS-group when compared to non-RS group (0% vs. 24%, p = 0.004 for acute GI toxicity; 4% vs. 33%, p = 0.003 for late GI toxicity). There were no reported acute or late grade 2 or above GI toxicities.Conclusions: Insertion of RS in men treated with LDR-BT is safe and resulted in a significant reduction in rectal dosimetry. The reduction in rectal dosimetry with RS insertion translates into significantly reduced acute and late GI toxicities.