Background: Retroperitoneal teratomas are rare extragonadal germ cell tumors of childhood that are diagnosed predominantly during the first year of life. Although mature forms are histologically benign, their close anatomical relationships with major retroperitoneal vessels and adjacent organs may render surgical resection technically demanding.

Case Presentation: An 11-year-old girl presented with a two-month history of lumbar and epigastric pain and a palpable, firm mass in the upper abdomen. Contrast-enhanced computed tomography demonstrated a well-circumscribed retroperitoneal mass measuring approximately 8 × 7 × 6 cm, situated posterior to the body and tail of the pancreas and containing cystic, fatty, calcified, and solid components. The lesion compressed and displaced the superior mesenteric artery and vein, the splenic vessels, the inferior mesenteric vein, the portal vein, the abdominal aorta, and the celiac trunk, with no imaging evidence of vascular invasion. Serum alpha-fetoprotein and carbohydrate antigen 19-9 levels were within reference limits. Through an upper midline laparotomy, careful vessel-controlled, stepwise perivascular dissection enabled complete excision of the tumor with an intact capsule despite its complex vascular relationships, while preserving the pancreas and all major vascular structures; neither vascular resection nor reconstruction was required. Histopathological examination confirmed a mature retroperitoneal teratoma (dermoid cyst). The postoperative course was uneventful, with no complications. At the one-month follow-up after discharge, the patient was clinically well, and abdominal ultrasonography revealed no abnormalities.

Conclusions: A mature retroperitoneal teratoma may combine benign biological behavior with considerable surgical complexity. In lesions situated posterior to the pancreas, detailed preoperative cross-sectional imaging and meticulous stepwise perivascular dissection may allow complete tumor excision without pancreatic or vascular resection when uninvolved structures can be safely preserved.