Olfactory neuroblastoma (ONB) is an unusual malignant neoplasm originating from the olfactory neuroepithelium. Secretion of adrenocorticotropic hormone (ACTH) from this tumor has been exceptionally reported. We describe a young man with resistant hypertension and a cushingoid phenotype. After hormonal confirmation of an ACTH-dependent Cushing syndrome, non-invasive dynamic tests were carried out to evaluate the cause of the ACTH source. Plasma cortisol decrease after a high-dose dexamethasone suppression test and cortisol increase after a desmopressin (DDAVP) stimulation test suggested a Cushing disease. A magnetic resonance image (MRI) of the brain and an Indium-111 octreotide scan revealed a large mass centered in the sphenoid sinus with lateral and posterior extension. An ACTH secreting ONB was confirmed with a trasnasal biopsy. Patient was offered a combined therapy with surgical resection and radiotherapy but refused surgery. The neoplasm was treated with neoadjuvant cisplatin-based chemotherapy followed by fractionated radiotherapy. Hypercortisolism initially improved with metyrapone but normocortisolism was only achieved after local control of the tumor with radiotherapy. Clinical presentation of ONB is usually related to local symptoms (as nasal obstruction and epistaxis) dependent on its ubication and extension. Cushing syndrome from ACTH production is a rare manifestation of ONB. This case also underlies the difficulties related to the interpretation of dynamic endocrine tests in Cushing syndrome.