Introduction: Primary adenocarcinoma of the appendix is a rare and heterogeneous malignancy, most commonly presenting as mucinous adenocarcinoma. Due to its nonspecific clinical manifestations, the diagnosis is often established at an advanced stage, frequently with already present peritoneal dissemination. Standard treatment for advanced mucinous appendiceal carcinoma involves cytoreductive surgery with HIPEC followed by systemic chemotherapy, or initial systemic therapy for downstaging, followed by repeat CRS and HIPEC. In cases where radical surgery is not feasible, the average survival is less than one year. Case report: We report a case of a 72-year-old woman with no relevant family history, who presented with acute right lower abdominal pain and distension. Imaging revealed multiple cystic peritoneal implants, ascites, and small bowel obstruction. Palliative emergency surgery demonstrated an appendiceal mass invading the right ovary, with widespread peritoneal mucinous deposits. Histopathological analysis confirmed mucinous adenocarcinoma of the appendix with peritoneal carcinomatosis. Postoperatively, the patient received 12 cycles of bevacizumab plus FOLFOX chemotherapy, achieving marked radiologic and biochemical improvement. Maintenance therapy with bevacizumab plus 5-FU/LV led to sustained disease stabilization. After 32 cycles, the patient developed an entero-vaginal fistula, which was surgically managed due to poor quality of life. Intraoperatively, the decision was made for cytoreductive surgery and fistulectomy. Postoperative imaging showed no evidence of disease. The patient remains progression-free for nearly three years, and disease-free for one year. Conclusion: This case highlights the potential for long-term survival in initially inoperable patients with advanced mucinous appendiceal adenocarcinoma and peritoneal carcinomatosis when treated with combined cytoreductive surgery and bevacizumab plus chemotherapy. Multimodal and individualized treatment strategies may significantly improve outcomes in this rare malignancy.

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