Introduction: Mucinous anal adenocarcinoma is a rare and often misleading entity, because its presentation may mimic suppurative perineal disease [abscess/fistula]. We report a case of fistulizing mucinous anal carcinoma diagnosed by endoscopic ultrasound and fine-needle aspiration, and discuss the diagnostic and therapeutic aspects. Case presentation: A 73-year-old man with a history of surgery for an anal fistula [2000] presented with purulent anal discharge associated with mild anal incontinence, without rectal bleeding or bowel habit disorders, and with preserved general condition. Proctological examination found fistulous openings and sphincter hypotonia. Pelvic MRI showed a mass in the left ischioanal fossa fistulizing toward the anal canal. Rectal endoscopic ultrasound showed a heterogeneous pararectal mass measuring 33 × 47 mm, with a fluid component and sparing the rectal wall; histological examination of the fine-needle aspiration sample confirmed mucinous adenocarcinoma. The staging work-up found no secondary lesions. A multidisciplinary decision was made to proceed with chemoradiotherapy, followed by reassessment and surgical discussion according to the clinical course. Conclusion: In the presence of a chronic, recurrent, or complex anal fistula, particularly when associated with a mass, induration, or functional impairment, malignant degeneration should be considered and documented by tissue sampling. MRI and endoscopic ultrasound with fine-needle aspiration/biopsy guide the diagnosis. Management is often multimodal and resembles that of rectal adenocarcinomas, with discussion of neoadjuvant treatment followed by radical surgery such as abdominoperineal resection.