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Marjolin's ulcer (MU) is a rare but aggressive squamous cell carcinoma arising from chronic cutaneous injury and has been only exceptionally reported in domestic animals. Chronic skin wounds are common in small ruminants raised under semi-arid conditions and may predispose to malignant transformation; however, MU has not previously been documented in sheep. Here, we report five cases in Santa Inês ewes from a confined flock in the semi-arid region of Northeastern Brazil. The affected 36-month-old ewes developed chronic dorsal wounds after repeatedly crawling under galvanised wire fencing, resulting in persistent mechanical lacerations over a two-year period. These non-healing hypertrophic wounds enlarged rapidly during the three months preceding clinical evaluation. The lesions were multinodular, ulcerated, friable, and malodorous, with extensive scarring consistent with prolonged secondary-intention healing. Histopathology revealed a moderately differentiated infiltrative squamous cell carcinoma (SCC) arising from ulcerated epidermis and characterised by marked keratinocyte atypia, keratin pearl formation, fibrosis, and chronic fibrinosuppurative dermatitis. Immunohistochemistry demonstrated diffuse pancytokeratin immunolabelling, confirming the epithelial origin of the neoplasm. Complete necropsies of all five affected ewes revealed no evidence of lymphatic or visceral metastases. This investigation represents the first documented report of traumatic MU in sheep associated with chronic dorsal fence-wire injury and underscores the importance of distinguishing this entity from solar radiation-induced SCC, which typically affects non-pigmented, sun-exposed anatomical sites. Early recognition and prompt management of chronic wounds are essential to prevent malignant transformation and improve outcomes in small ruminants.