Appendiceal mucinous adenocarcinoma is a rare gastrointestinal malignancy that may metastasize to the ovaries and closely mimic advanced primary ovarian cancer, creating diagnostic and therapeutic challenges. A 61-year-old postmenopausal woman presented with postmenopausal bleeding, abdominal distension, abdominal pain, and weight loss. Imaging demonstrated bilateral adnexal masses with omental caking and ascites, suggestive of advanced ovarian malignancy. Cancer antigen 125 was elevated, and carcinoembryonic antigen (CEA) levels were markedly increased. She had a history of acute appendicitis 1 year earlier. Biopsy revealed mucinous adenocarcinoma with signet-ring cells. Endoscopic evaluation was unremarkable. Despite neoadjuvant chemotherapy for presumed ovarian cancer, the disease progressed, necessitating cytoreductive surgery. Histopathological examination demonstrated bilateral mucinous adenocarcinoma with peritoneal spread. Immunohistochemistry showed positivity for cytokeratin 20, caudal-type homeobox 2, and special AT-rich sequence-binding protein 2 and negativity for cytokeratin 7 and paired-box gene 8, confirming metastatic appendiceal origin. She was started on fluorouracil, leucovorin, and oxaliplatin chemotherapy. Metastatic appendiceal carcinoma can closely resemble primary ovarian malignancy. Bilateral mucinous ovarian tumors with elevated CEA levels and poor chemotherapy response should prompt evaluation for a gastrointestinal primary tumor.