Introduction Duodenal teratoma is a rare condition with only four cases reported in the English literature. Radiological imaging and tissue sample are necessary for diagnosis in addition to tumor markers. The most effective treatment is still complete excision with safety margins. Case presentation We report a case of 26 years-old-man, in whom epigastric pain, decreased appetite, and postprandial bilious vomiting had been prevalent for 5–6 months and had exacerbated prior to the emergency room. Enhanced abdominal computed tomography revealed a 10 × 15cm heterogeneous solid mass with cystic component in the third duodenum segment. The inferior veina cava and aorta were both compressed, although there was no sign of lymphadenopathy or ascites. An ulcerating non-bleeding lesion at the D2-D3 junction of the duodenum was discovered during a gastroduodenoscopy. Biopsies and immunohistochemical investigations revealed findings that were consistent with a mixed non-seminomatous germ cell tumor. A PET-CT scan was performed, which revealed FDG uptake by the duodenal lesion but no evidence of metastatic lesions. A distal duodenal segmentectomy is performed, and then a duodeno-jejunal anastomosis is used to restore continuity. The final diagnosis was teratomatous tumor of the duodenum without malignant changes. Conclusion This is the second adult case of main duodenal teratoma that has been reported. We publish it to encourage surgeons to think about this differential diagnosis and carefully plan surgery using a multidisciplinary approach.