We describe the case of a 28-year-old-male patient with rib trauma who presented an incidental imaging finding in a contrast-enhanced abdominopelvic computed tomography of a 6-cm cystic appearance lesion with mixed density and hypodensity in some areas and soft-tissue density in the areas in contact with the anterior border of the pancreas. The mass was also in contact with small bowel and mesenteric vessels. Potential differential diagnoses considered were solid-cystic papillary tumor or mucinous cystadenoma. A magnetic resonance imaging (MRI) was requested, which showed a heterogeneous cystic-looking 6-cm lesion with minimal peripheral calcifications in the omental bursa in close relation with the pancreatic tail with an apparent separation plane. The patient also reported a previous episode of melena (black stool), which raised the possibility of bowel duplication as differential diagnosis. The patient was admitted and underwent a scheduled resection surgery of a retroperitoneal cystic tumor. The patient was positioned in dorsal decubitus and the ports were placed as follows. Exploratory laparoscopy did not reveal any signs of peritoneal carcinomatosis or metastasis. First, the gastrocolic ligament was dissected with a Harmonic scalpel, preserving the vascular arcade of greater gastric curvature, which enabled access to the omental bursa. At the root of the mesentery, in proximity to the first jejunal loop, an approximately 6 cm diameter, firm, elastic, rounded lesion was identified. Resection was performed preserving the capsule. Due to its close proximity to the first jejunal loop and because of the impossibility of performing a safe resection, a supraumbilical midline incision was made. Once the cavity was opened, a complete resection of the tumor was performed, and the sample was sent to the pathology department for further study. The patient had a favorable postoperative progress. He remained at the institution for 36 h. Macroscopically, a cystic formation of 7.6 × 6 × 4 cm, with a smooth brownish external surface was identified. Upon sectioning, it showed a smooth whitish inner wall up to 0.3 cm of thickness, with friable whitish material inside. The deferred anatomic pathology study, upon microscopic examination, showed that the walls were formed by dense fibrous tissue. No muscle tissue compatible with an intestinal duplication cyst was identified. Occasional pigmented histiocytes and partially calcified amorphous material were observed on the inner surface. No epithelial lining was identified on the inner surface. No hyaline material consistent with a hydatid cyst was observed. The anatomic pathology diagnosis was mesenteric root pseudocyst.