Background <p>Thymic neuroendocrine tumors associated with ectopic adrenocorticotropic hormone (ACTH) syndrome are rare, and postoperative adrenal crisis presenting with atrial tachycardia as the initial manifestation is even more uncommon. As a result, evidence to guide early recognition and management remains limited.</p> Case presentation <p>This article reports a case of an ectopic ACTH-secreting thymic neuroendocrine tumor, complicated by postoperative adrenal crisis, with recurrent atrial tachycardia. Following tumor resection, the patient developed profound shock accompanied by recurrent atrial tachycardia, which improved significantly after prompt initiation of glucocorticoid administration and ventricular rate control.</p> Conclusions <p>Adrenal crisis often presents with nonspecific clinical symptoms, predisposing it to delayed or missed diagnosis. However, during the perioperative management of ectopic ACTH-secreting neuroendocrine tumors, failure to administer glucocorticoids promptly and adequately can trigger an adrenal crisis that may rapidly progress to life-threatening complications, including atrial tachycardia, shock, and acute respiratory failure.Therefore, clinicians should aim to recognize this risk at the earliest possible stage. A multidisciplinary approach during the perioperative period remains essential to ensure timely and individualized glucocorticoid replacement therapy, thereby optimizing patient outcomes.</p> Clinical trial number <p>Not applicable.</p>