Cushing's syndromes (CS) due to thymic carcinoids are rarely seen. In this text, a case with CS due to ectopic adrenocorticotropic hormone (ACTH) secreting atypical thymic carcinoid tumor is presented. A 50-year-old Turkish male patient was admitted to our emergency department with typical CS features. Basal hormone profile, low- and high-dose dexamethasone suppression tests, and inferior petrosal sinus sampling results were consistent with ectopic ACTH secretion. Thorax computerized tomography showed an upper mediastinal mass, and trans-thoracic biopsy showed atypical thymic carcinoid with positive ACTH staining. Since the vascular invasion was detected, tumor was accepted inoperable; somatostatine receptor analogs, chemotherapy, and radiotherapy were planned. Ectopic CS can be derived from atypical thymic carcinoid. In this case, ACTH staining was used to confirm ACTH secretion from thymic tissue, and positive staining was detected. ACTH staining routinely was not performed for extra hypophyseal tissue tumors. In suspicious and difficult cases, ACTH staining can be helpful to confirm the presence of ACTH in tumor tissues.