A 12-year-old male neutered Siamese cat presented with a history of inappetance and lethargy and an enlarged left anal sac. The anal sac was surgically excised and histopathology confirmed the diagnosis of anal sac adenocarcinoma. Perianal tumours are rare in the cat and anal sac adenocarcinoma has not been previously reported. This is in contrast to the dog where anal sac adenocarcinoma is a well recognised albeit uncommon tumour.