A 37-year-old man presented to our hospital for early rectal cancer accompanied by mucosal prolapse syndrome. Biopsy confirmed an adenocarcinoma, and endoscopic ultrasonography indicated proximity to the dentate line but no submucosal invasion. The tumor was removed en bloc via endoscopic submucosal dissection without complications, and its margin was free of tumor cells. The total procedure duration was 37 minutes, and the resected specimen measured 23 × 13 mm. There was no recurrence during the 3-year observation period. Although close to the dentate line and accompanied by mucosal prolapse syndrome, a rectal cancer lesion was safely resected en bloc using endoscopic submucosal dissection.