Purpose: Squamous cell carcinoma (SCC) arising in the mesorectum is exceptionally rare, with only one previously reported case. We describe a diagnostically challenging case of human papillomavirus (HPV)-associated SCC presenting as a recurrent cystic pelvic mass, highlighting the role of surgical resection in establishing diagnosis and guiding management. Methodology: A 54-year-old woman presented with progressive lower abdominal pain and imaging demonstrating a cystic pelvic collection initially managed as complicated diverticular disease. Despite antibiotics, drainage, and interval laparoscopy, the lesion recurred with evolving features and associated hepatic lesions. Ongoing diagnostic uncertainty prompted definitive surgical management with laparoscopic-assisted low anterior resection and en bloc resection of adjacent structures. Results: Histopathology demonstrated a cystic SCC within the mesorectum with focal invasion of the rectal wall but no mucosal involvement. Immunohistochemistry showed strong p16 positivity, and in situ hybridisation confirmed high-risk HPV 16/18, consistent with HPV-associated SCC. No primary tumour was identified on post-operative imaging, and the findings were most consistent with metastatic carcinoma of unknown primary origin. Regional lymph nodes were negative for malignancy. The patient recovered well post-operatively and was referred for systemic therapy. Conclusion: This case represents an exceptionally rare presentation of HPV-associated SCC manifesting as a cystic mesorectal mass. It underscores the diagnostic difficulty of recurrent pelvic collections and highlights the importance of surgical resection when malignancy cannot be excluded. SCC should be considered in the differential diagnosis of atypical cystic pelvic lesions, and multidisciplinary management is essential for optimal outcomes.