Anal adenocarcinoma is a rare condition and can arise in chronic inflammatory states such as in Crohn's disease, or in a chronic fistula-in-ano. We report our diagnosis and management of a patient who presented with a large perineal lump with a long-standing history of perianal fistulous disease. This was initially evaluated with a Magnetic Resonance Imaging, and the diagnosis was confirmed with biopsy. Multimodality treatment with chemoradiotherapy and surgery should be offered to achieve the best outcomes.