A 74-year-old Japanese woman presented with a 3-month history of anal bleeding. Proctoscopy revealed an unusual polypoid lesion with focal pigmentation at the dentate line, which was histologically diagnosed as a malignant melanoma. Whole-body clinical and radiographic evaluations revealed no alternative primary source. Endoscopic ultrasonography (EUS) showed well-delineated hypoechoic tumors invading the muscularis propria, and magnetic resonance imaging (MRI) revealed regional lymphadenopathy. Following this evaluation, an abdominoperineal resection with regional lymphadenectomy was performed. The excised tumor was histologically confirmed to be malignant melanoma, and its depth and metastatic lymph nodes proved to have been accurately and precisely evaluated by the preoperative examinations. Thus, EUS and MRI are useful preoperative diagnostic tools for the tumor staging of primary anorectal malignant melanomas, as for other rectal tumors.