A case of cesarean scar endometriosis with massive decidualization is presented. The 25-year-old patient had an extensive, ulcerated lesion that mimicked malignancy microscopically due to myxoid change with alveolar patterns reminiscent of some soft tissue sarcomas, signet ring-like cells similar to mucin-producing carcinoma, and pseudoinfiltration of the fascia. The myxoid tissue was positive for acid mucopolysaccharides but negative with PAS. Decidual cells were vimentin positive and keratin negative. No atypia or mitoses were seen. The pseudoinfiltrative aspect was due to abundant extracellular matrix that separated the fascicles of the fascial tissue. There was a metaplastic decidual "proximity effect" in the surrounding, unaffected dermis, which may be responsible for the expansile features of the lesion. This case exemplifies the differential diagnosis of myxoid endometriosis with malignant conditions of the skin.