Ectopic corticotropin-releasing hormone (CRH) syndrome, a rare subtype of adrenocorticotropic hormone-dependent Cushing syndrome, is associated with tumors of diverse origins. Here, we present a case of a 37-year-old female diagnosed with ectopic CRH syndrome secondary to rectal large cell neuroendocrine carcinoma, a hitherto unprecedented site for CRH-secreting tumors. The patient presented with classical features of Cushing syndrome, supported by laboratory evidence of hypercortisolemia and disrupted diurnal cortisol secretion. Imaging studies ruled out a pituitary adenoma, whereas colonoscopy identified a rectal malignancy. Immunohistochemical staining confirmed the presence of ectopic CRH syndrome. Despite prompt chemotherapy initiation, the patient's condition rapidly deteriorated, highlighting the aggressive nature and dismal prognosis associated with rectal large cell neuroendocrine carcinoma linked to ectopic CRH syndrome. This case underscores the importance of early recognition and comprehensive management to optimize patient outcomes.