A 65-year-old woman who had been diagnosed with dermatomyositis presented to the hospital with a small bowel mass. She had tested positive for fecal occult blood test and anemia at a medical checkup;therefore, computerized tomography (CT) was performed at the previous hospital and it had revealed thickening of the intestinal wall. Abdominal contrast-enhanced CT, single-balloon assisted enteroscopy, and biopsy led to a diagnosis of poorly differentiated jejunal adenocarcinoma. The patient underwent laparoscopic segmental resection of the jejunum with dissection of mesenteric lymph nodes. A histological examination revealed that the tumor was neuroendocrine carcinoma (NEC), large-cell type of the jejunum, pT3, pN0, sM0, and pStage IIA. Immunohistochemically, the NEC component was positive for chromogranin A and negative for neural cell adhesion molecule and synaptophysin. The MIB-1 index was 60%. Four courses of postoperative chemotherapy using cisplatin and etoposide were administered. The patient is currently doing well without any recurrence or metastasis. To the best of the author's knowledge, this is the first report of dermatomyositis associated with primary jejunal NEC.