Background We previously reported the results of a phase 3 trial evaluating a prostate/rectal hydrogel spacer during prostate intensity modulated radiation therapy, which resulted in decreased rectal dose and toxicity and less decline in bowel quality of life (QOL). A secondary analysis was performed to correlate penile bulb dose and sexual QOL. Methods and materials Sexual QOL was measured with the Expanded Prostate Cancer Index Composite (EPIC) by mean scores, the proportion of patients with a minimal clinically important difference (MID), and analyses of the different items composing the sexual domain. Results A total of 222 men enrolled with median follow-up of 37 months. Hydrogel reduced penile bulb mean dose, maximum dose, and percentage of penile bulb receiving 10 to 30 Gy (all P <.05) with mean dose indirectly correlated with erections sufficient for intercourse at 15 months (P =.03). Baseline EPIC was low (53 [standard deviation ± 24]) with no difference between arms (P >.1). A total of 41% (88/222) of men had adequate baseline sexual QOL (EPIC >60 (mean, 77 [± 8.3]). This subgroup at 3 years had better sexual function (P =.03) with a spacer with a smaller difference in sexual bother (P =.1), which resulted in a higher EPIC summary on the spacer arm (58 [±24.1] vs control 45 [± 24.4]) meeting threshold for MID without statistical significance (P =.07). There were statistically nonsignificant differences favoring spacer for the proportion of men with MID and 2× MID declines in sexual QOL with 53% vs 75% having an 11-point decline (P =.064) and 41% vs 60% with a 22-point decline (P =.11). At 3 years, more men potent at baseline and treated with spacer had “erections sufficient for intercourse" (control 37.5% vs spacer 66.7%, P =.046) as well as statistically higher scores on 7 of 13 items in the sexual domain (all P <.05). Conclusions The use of a hydrogel spacer decreased dose to the penile bulb, which was associated with improved erectile function compared with the control group based on patient-reported sexual QOL.