A 60-year-old man presented with an acute confusional state 2 months after parotidectomy for dedifferentiated adenocarcinoma of the parotid gland with nodal metastasis. Investigation indicated ectopic adrenocorticotropic hormone (ACTH) production from metastatic parotid carcinoma resulting in Cushing syndrome. Parotid gland adenocarcinoma is an extremely rare source of ectopic ACTH production with only 2 cases previously reported in the literature, both of which were conventional acinic cell carcinoma. This is a first case report of a dedifferentiated acinic cell carcinoma causing ectopic ACTH syndrome.