PURPOSE: Hidroacanthoma simplex (HAS) is a rare benign neoplasm of the skin and known as intraepidermal poroma. Malignant transformation of HAS has been reported in the literature, but it is very rare. We experienced a case of malignant hidroacanthoma simplex (MHA) and performed the surgery of the neoplasm. METHODS: A 73-year-old patient visited our department for evaluation and treatment of the skin lesion (3.2 x 3.0 cm2) that occurred two years before on the right ankle area. The lesion was characterized by dark-brown, hyperkeratotic and protruding patch. The patient had no symptom, and any palpable lymph node was not found on physical examination. The result of laboratory was also nonspecific. The lesion was excised completely with a resection margin of 5 mm including subcutaneous tissue partially, followed by full-thickness skin graft for reconstruction. RESULTS: The result of biopsy was reported as malignant hidroacanthoma simplex. Histologically, the epidermis showed discrete aggregates of tumor cells which had round, hyperchromatic nuclei and pale or clear abundant cytoplasm. Cystic spaces were found within the tumor and mitosis were seen, frequently. No recurrence was found one month after surgery. CONCLUSION: The case of malignant hidroacanthoma simplex was treated with full-thickness skin graft following wide excision of the tumor.