Primary malignant melanoma originating in the digestive tract is extremely rare. A case of primary malignant melanoma in the descending colon is described. The tumor was an elevated mass with surface necrosis. Histologically, tumor cells were arranged with compact nests surrounded by fibrous stroma. The tumor cells had pleomorphic nuclei and rich cytoplasm. In some areas, cells of signet ring-like appearance were found. An immunohistochemical examination showed that most of the tumor cells were positive for S-100 protein, HMB-45, melan-A, vimentin and CD38. Ultrastructural examination confirmed some premelanosomes. EWS-ATF-1 fusion transcript, which is usually detected in clear cell sarcoma, was not demonstrated on reverse transcriptase-polymerase chain reaction. Because there was no evidence of either cutaneous or ocular primary melanoma, the tumor was thus diagnosed as primary colonic malignant melanoma. The patient has remained free of recurrent disease for 3 years after a surgical resection. Colonic malignant melanoma must be differentiated from other intestinal tumor, and the possibility of metastasis from another more common primary site must be ruled out.