Ectopic ACTH-secreting pituitary adenoma, arising outside the sella turcica from residual cells of Rathke's pouch, is an exceptionally rare cause of Cushing's syndrome. Diagnosis is often challenging and may be late despite extensive clinical and biochemical work-up. We report here an ectopic corticotroph tumor of the maxillary sinus, which was finally localized by  ¹⁸ F-FDG PET/CT and somatostatin receptor scintigraphy. A 38-year-old woman was referred for suspicion of ACTH-dependent Cushing's syndrome. Biological testing was indicative of ectopic ACTH secretion; however, extensive investigation failed to identify any culprit tumor. In contrast, pituitary MRI revealed a doubtful 4-mm right-side pituitary lesion, leading to hypophysectomy, without clinical or biochemical remission. After pituitary surgery, medical therapy was initiated but had limited efficacy, and bilateral adrenalectomy was subsequently performed. Following the adrenalectomy, the patient developed hyperpigmentation due to elevated ACTH levels. ¹⁸ F-FDG PET/CT and somatostatin receptor scintigraphy (Octreoscan®), years after initial diagnosis, revealed a hypermetabolic lesion in the maxillary sinus. Surgical resection identified an ectopic ACTH-secreting pituitary adenoma expressing ACTH and T-Pit; a marked decrease in plasma ACTH was observed postoperatively. In conclusion, we report a case of ACTH-dependent Cushing's syndrome, caused by an ectopic corticotroph adenoma located in the maxillary sinus. This case illustrates the diagnostic challenges in localizing ectopic ACTH-secreting pituitary adenomas and highlights the value of nuclear medicine imaging in identifying these unusual lesions.