A mesenteric cyst is a rare, mostly benign, intra-abdominal tumor. Treatment is almost always surgical and consists of removing the cyst and involved organs whenever possible to prevent recurrence. The following is the case of a 38-year-old patient with nonspecific abdominal pain and an ultrasound diagnosis of a giant retro-peritoneal mass in the left medial paracolic gutter. The following imaging studies were performed: computed tomography, magnetic resonance, and endoscopy, finding a giant cystic lesion. An exploratory laparotomy was performed to remove the mass, and a histopathology report confirmed the diagnosis of mesenteric cyst.