Gardner's syndrome (GS) is a rare syndrome with autosomal dominant inheritance, which is characterized by multiple intestinal polyps, dental anomalies, desmoid tumors, and soft tissue tumors. All gastrointestinal symptoms seen in GS are associated with the underlying familial adenomatosis polyposis and abdominal desmoid tumors, with the most common symptoms being anemia, lower gastrointestinal bleeding, abdominal pain, diarrhea, obstruction, and mucous defecation. To our best knowledge, no case of GS that has presented with gastrointestinal perforation and bleeding has ever been reported in the English language medical literature. A 37-year-old male who had been diagnosed with GS five years earlier was referred to our clinic for lower gastrointestinal bleeding. Despite the absence of a bleeding focus on conventional angiography, the patient was operated on with laparotomy, due to the persistence of both signs and symptoms of mild peritonitis. On the laparotomy, the patient was noted to have areas of perforation in the duodenum, splenic flexura, and midrectum. The third and fourth part of the duodenum, the proximal 15 cm segment of the jejunum, a 10 cm segment of the terminal ileum, the whole colon, and the upper and middle rectum were resected, and duodenojejunal side-to-side anastomosis and terminal ileostomy were performed. The histopathological analysis of the large mass measuring 30 cm x 20 cm was reported as a desmoid tumor. The pathological examination of the tumor foci detected in the colonic specimen revealed poorly differentiated adenosquamous carcinoma.