A 67-year-old female who had undergone hysterectomy and colpectomy due to vaginal premalignant melanosis presented with sacralgia after 2-year discontinuation of follow-up. Mucosal pigmentation was observed around the external urethral orifice, strongly implying recurrence of premalignant melanosis. Magnetic resonance imaging for abdominal screening revealed a cystic mass adjacent to the urinary bladder with heterogeneous internal signal measuring 5 cm in diameter, but no definitive diagnosis could be made. She underwent urethrectomy and cystostomy, with simultaneous extirpation of the intrapelvic cystic tumor. The mucosal pigmentation included atypical melanocytes, and histologically the intrapelvic cystic tumor containing melanin-rich fluid demonstrated existence of malignant melanocytes. She received adjuvant DAV-Feron immunochemotherapy using dacarbazine, nimustine, vincristine and interferon-beta. She has been followed up for 14 months without any evidence of local recurrence or metastasis.