Sarcomatoid carcinoma (SC) is a rare malignant tumor with properties of both epithelial and mesenchymal carcinomas. SC has been reported in various organs, but the number of reports for each type is small. Small intestinal tumors make up about 3-6% of gastrointestinal malignancies. Discovering them in the early stage is rare and difficult, with anemia and/or abdominal pain as the major symptoms of small intestinal tumors. Primary small intestinal SC (SISC) is rare among small intestinal tumors, and currently very few cases have been reported in the literature. Previous studies have reported that neither chemotherapy nor radiotherapy improves the overall survival rate of patients with SISC, and the prognosis is extremely poor. Currently, surgical resection remains the only optimal therapeutic approach for SISC. Here, we present the case of a 90-year-old woman who had acute peritonitis due to perforation of a small intestinal tumor. She underwent emergency exploratory laparotomy and partial resection of the small intestine, including the tumor. The tumor was pathologically identified as a primary SISC with mesenteric lymph node metastasis. Subsequently, she had recurrence in the intra-abdominal area and lymph node metastasis anterior to the inferior vena cava and died 15 months after surgery without any additional treatment.