Necrotizing fasciitis is a lethal rapidly progressive bacterial infection characterized by widespread skin, subcutaneous and muscular tissue necrosis. In rare cases could be secondary to colocutaneous fistula, an abnormal communication between colon and skin which can occur either spontaneously or after trauma or after surgical procedures, procedures, particularly in cancer patients. A 73-year-old man presented to the Emergency Department with preshock symptoms. He had a previous history of rectal cancer treated with low anterior rectal resection and loop colostomy followed by adjuvant radiotherapy and ostomy closure complicated three months later by a Fournier's gangrene for which he underwent debridement, drainage and left orchiectomy. Five days after admission a computed tomography scan demonstrated a colocutaneous fistula from rectum to left thigh associated with signs of necrotizing fasciitis. Therefore, a left thigh debridement, left hemicolectomy, terminal colostomy and Vac-therapy placement were performed. Though repeated Vac-therapy changes, aimed antibioticotherapy, continuous vascular and respiratory support the patient died on 41 postoperative day. Necrotizing fasciitis of the thigh secondary to colocutaneous fistulae represents a very rare and challenging condition, even more in rectal cancer setting. Early diagnosis is crucial to improve management and outcome. Treat-ment management should include antibiotics, intensive supportive care and aggressive surgical treatment.(Cite this article as: Siragusa L, Sforza D, Sensi B, Pathirannehalage Don C, De Luca L, Ciancio Manuelli M, et al. Left thigh necrotizing fasciitis secondary to colocutaneous fistula in previously treated rectal cancer: case report of a single, late and fatal complication. Chirurgia 2023;36:43-6. DOI: 10.23736/S0394-9508.22.05422-5)