PURPOSE: To determine the impact of sarcopenia on erectile functional outcomes after a nerve-sparing (NS) robot-assisted radical prostatectomy (RARP) using patient-reported validated questionnaires. MATERIALS AND METHODS: In this retrospective study, RARP was performed on 841 patients at Okayama University Hospital, of which 132 underwent NS RARP. Erectile functional outcomes were assessed using the 5-item version of the International Index of Erectile Function (IIEF-5) and the Expanded Prostate Cancer Index Composite before and 1, 3, 6, and 12 months after surgery. Automated measurement of skeletal muscle at L3 was achieved using volume analyzer software and normalizing for height (cm²/m²) to calculate skeletal muscle index (SMI). Patients who had an IIEF-5≤4 comprised the group with erectile dysfunction (ED), and those with an IIEF-5≤5 made up the non-ED group. RESULTS: This study enrolled 95 patients of median age 65 years with a preoperative IIEF-5 of 16. There were no significant differences between patients with and without sarcopenia among those with preoperative IIEF-5. Postoperatively, in the ED group, SMI and preoperative IIEF-5 were significantly lower than in the non-ED group. Multiple linear regression analysis revealed that (1) both SMI and preoperative IIEF-5 were independent predictors of ED, and (2) sarcopenia and preoperative IIEF-5 were predictors of ED at 12 months after NS RARP. CONCLUSIONS: Patients with sarcopenia can have worse erectile functional outcomes after NS RARP. Sarcopenia and a lower preoperative IIEF-5 score may be predictive of postoperative ED.