A 55-year-old woman presented with a mucopurulent sinusal discharge from the right supragluteal region, with symptoms over the previous five months. This abscess began as a slowly swelling growth, which eventually turned into a discharging sinus, and she was diagnosed with a gluteal abscess. The patient underwent incisional drainage, and intra-operatively, the sinus tract could be seen extending to the retroperitoneum. A subsequent CT scan and an MRI of the abdomen revealed a large heterogeneous retroperitoneal cystic mass on the right side of midline, extending inferiorly into the anterior thigh along the iliopsoas. Superiorly, a tubular projection extended from the lesion, indenting the ileocaecal junction, while a fluid filled cutaneous fistulous tract was seen, extending to the right flank. A diagnosis of pseudomyxoma retroperitonei, likely of retrocaecal appendicular origin, was proposed. An explorative laparotomy with an appendectomy, and the evacuation of the retroperitoneal collection were completed. The subsequent histopathology confirmed the diagnosis of appendicular mucinous cystadenoma, with pseudomyxoma retroperitonei.