We report a very unusual case of malignant sigmoidoduodenal communication. To the best of our knowledge, this is the first documentation of this entity in the English language literature. A 76-year-old man presented with weakness, severe weight loss, foul-smelling eructations, anemia, constipation, and episodes of diarrhea. A sigmoidoduodenal fistula was found by barium enema, and a diagnosis of ulcerative colonic adenocarcinoma was made from the colonoscopy findings. Thus, we performed sigmoid colectomy with resection of the fistula and the involved anterior wall of the third duodenal part, followed by primary closure of the duodenal defect. Histological examination confirmed a Dukes' B (Stage II - T(4)N(0)M(0)) colonic adenocarcinoma, and the excision margins of the resected duodenal specimen were clear. We gave adjuvant chemotherapy with 5-fluorouracil and leucovorin. The patient is still alive and disease-free, 2 years postoperatively.