Objective: This systematic review was performed to evaluate the efficacy and safety of phosphodiesterase-5 inhibitors (PDE5i) in the treatment of erectile dysfunction (ED) after radiotherapy for prostate cancer (PCa). Methods: A systematic search of PubMed, Embase and the Cochrane Library was performed to identify all randomized controlled trials (RCTs). All relevant studies on the outcomes and complications of PDE5i in the treatment of ED after radiotherapy for PCa were assessed. The outcomes and complications analyzed for this study included the International Index of Erectile Function (IIEF) questionnaire, Global Efficacy Questions (GEQs), Sexual Encounter Profile (SEP) diary and side effects. The Cochrane Collaboration Review Manager software (RevMan 5.1.4) was used for statistical analysis of the outcomes and complications. Results: A total of 4 RCTs were identified from the search strategy. Compared with placebo, the trials indicated that PDE5i significantly improved the IIEF scores, with the exception of two questions (questions 6 and 13), and statistically significantly more positive answers for the GEQs and SEP diary were acquired in intervention groups. Furthermore, almost all the side effects in both groups were mild or moderate, transient and well tolerated. Except for headache, flushing and dyspepsia, all other adverse events did not differ significantly between the two groups. Conclusions: The systematic review suggested significant advantages in the efficacy and safety of PDE5i in the treatment of ED after radiotherapy for PCa. PDE5i should be considered as the first choice for the treatment of PCa patients with ED after radiotherapy.