Epidermal cysts are common, benign, intradermal or subcutaneous, typically asymptomatic masses, ranging from 1 to 4 cm in size. They may occur anywhere in the body, with a predilection for the face, neck, and trunk. Transformation to squamous-cell carcinoma is rare. We present a case of a 61-year-old male patient with a large, growing mass in his posterior left gluteal region. Given the concern for a malignancy, he was referred to a surgical oncologist. Magnetic resonance imaging (MRI) without contrast was performed due to poor renal function and revealed a large cystic mass in the left gluteal subcutaneous soft tissues that was subsequently excised. Pathological examination revealed an epidermal inclusion cyst that measured 17.8 × 13.18 × 5.8 cm. To our knowledge, this is the largest epidermal inclusion cyst reported in the English literature.