A 74-year-old man experienced vertical diplopia. Two years earlier, he was diagnosed with a squamous cell carcinoma of the periorbital frontal skin, with perineural spread involving the ophthalmic division of the right trigeminal nerve and the right facial nerve. The clinical findings were consistent with a right fourth cranial nerve palsy. Computerized tomography and magnetic resonance imaging demonstrated a discrete mass involving the belly of the right superior oblique muscle. An anterior orbitotomy and biopsy demonstrated a mass extending into the belly of the superior oblique muscle. Histology revealed an infiltrating squamous cell carcinoma. The possibility of perineural, direct, or metastatic spread to the superior oblique muscle should be considered in a patient with a history of squamous cell carcinoma of the head and neck. The authors believe this case to be the first report of superior oblique underaction due to involvement of the muscle by squamous cell carcinoma, presumably because of perineural spread. Diagnosis was made possible by neuroimaging and histopathology. There was good short-term resolution of the patient's diplopia after radiotherapy.