Introduction: Mucinous neoplasms of the appendix are rare, and mucinous adenocarcinoma arising from an appendiceal remnant is exceptionally uncommon, with only a limited number of cases reported in the literature. Case: We report the case of a 51-year-old male who underwent laparoscopic resection of a large retroperitoneal cyst. He had previously undergone an open appendectomy at the age of 23 for acute appendicitis, with histopathology demonstrating suppurative appendicitis. Seven years prior to cyst excision, a computed tomography (CT) scan performed for investigation of lower back pain incidentally identified a right-sided retroperitoneal cyst with benign features. Interval CT imaging over seven years demonstrated progressive enlargement of the cyst, with the development of calcifications and associated soft tissue thickening, raising concern for a cystic neoplasm. The patient underwent elective laparoscopic resection and had an uncomplicated postoperative recovery. Histopathological examination revealed a cyst wall composed of a double muscular layer lined by tall columnar epithelium. A moderately differentiated adenocarcinoma was identified, invading the muscularis propria. The patient remains under oncological surveillance with no evidence of recurrence to date. Discussion: Primary mucinous adenocarcinoma arising in the retroperitoneum is exceedingly rare. While malignant transformation of an isolated enteric duplication cyst is a possible aetiology, the anatomical location and prior appendectomy suggest origin from an appendiceal remnant. This case highlights the malignant potential of enlarging retroperitoneal cystic lesions and supports the need for long-term surveillance with cross-sectional imaging and endoscopic assessment. Reference: 1. Fujii Y, Sugimoto A, Nishijima R, et al. J Surg Case Rep. 2024;10:144.