Malignant melanoma of the lacrimal sac is very rare and primary malignant melanoma is extremely rare. It is usually diagnosed at an advanced stage after excision or biopsy of a tumor. We treated a patient with tearing and bloody discharge from the left eye. We performed a dacryocystectomy with the suspicion of a chronic dacryocystitis. However, the pathological findings and the immunohistochemical studies showed a malignant melanoma of the lacrimal sac. The patient underwent postoperative irradiation therapy. Follow up two months after surgery revealed no evidence of recurrence. Early diagnosis is very important for prognosis in patients with malignant melanoma of the lacrimal sac. Because this tumor often presents with symptoms similar to dacryocystitis and may masquerade as a chronic dacryocystitis, it can be difficult to make an early diagnosis.