Platelet-rich plasma (PRP) is used for tissue repair and regeneration. Herein, we investigated the therapeutic efficacy and mid-term durability of injections of PRP into the urethral sphincter for the management of postprostatectomy incontinence (PPI). Thirty-nine patients with PPI that were refractory to conservative treatments were prospectively enrolled. They received repeated PRP urethral sphincter injections monthly for a total of four months. The primary endpoint was the Global Response Assessment (GRA) score after treatment. The secondary endpoints included changes in the stress urinary incontinence (SUI) visual analog scale (VAS) from baseline to the end of follow-up and urodynamic parameters from baseline to 3 months. The mean follow-up period after the entire treatment course was 21.0 ± 11.3 (range: 1.6–36.3) months. After PRP injections, the median GRA score with quartiles was 2.0 (1.0, 2.0). The SUI VAS and abdominal leak point pressure significantly improved from 6.9 ± 1.8 to 4.4 ± 2.3, p <0.001, and from 74.8 ± 37.0 to 115.5 ± 57.9 cmH2O, p = 0.004, respectively, after the fourth PRP urethral sphincter injection. Following PRP urethral sphincter injections, the severity of SUI significantly reduced, indicating efficacy and mid-term durability as a novel treatment for PPI.