BACKGROUND Mixed adenoneuroendocrine carcinoma of the gallbladder (gMANEC) is an extremely rare cancer. Most of the cases are reported in Asia, North America, and Europe, with no cases reported in Latin America; this is the first report for this region, and the 24th case reported worldwide. CASE REPORT A 68-year-old woman was referred to our department due to asthenia and moderate abdominal pain in the right upper quadrant for 6 months, with imaging examinations showing a solid heterogeneous expansive lesion in gallbladder topography and segment IV of the liver. The MRI displayed an expansive and heterogeneous lesion with inaccurate limits in the gallbladder affecting segment IVb of the liver, in addition to lymphadenopathy in the hepatic hilum. A cholecystectomy with resection of segments IV-B and V of the liver (radical cholecystectomy) and hepatic hilar lymphadenectomy were performed. Anatomopathological examination and immunohistochemistry confirmed a primary mixed adenoneuroendocrine carcinoma of the gallbladder. The patient received adjuvant chemotherapy and radiotherapy; however, after the patient reported experiencing low back pain, a CT was performed, revealing retroperitoneal metastasis, and the radiotherapy was interrupted. Currently, the patient has a stable disease, following a protocol of 5-Fluorouracil and somatostatin, and she reports having low back pain of low intensity. CONCLUSIONS This is the 24th gMANEC case reported in the literature. The tumor was successfully resected; however, the patient presented retroperitoneal metastasis 6 months after surgery, despite combined adjuvant therapy.