We reported a unique case of posttraumatic giant infratentorial extradural intradiploic epidermoid cyst. A 54-year-old male, with a previous history of an open scalp injury and underlying linear skull fracture in the left occipital region in childhood, presented with a painful subcutaneous swelling, which had been developed gradually in the same region and moderate headache, nausea, vomiting and cerebellar ataxia. The duration of symptoms on admission was 3 months. Imaging studies revealed occipital bone destruction and giant extradural intradiploic lesion. The preoperative diagnosis was giant infratentorial extradural intradiploic epidermoid cyst. Surgery achieved total removal of the lesion, which was histologically confirmed and the postoperative course was uneventful. To our knowledge, this is the first case of giant infratentorial extradural intradiploic epidermoid cyst with a traumatic etiology described in the literature.