Background <p>Harlequin syndrome is characterized by unilateral facial flushing and sweating induced by exercise, heat, and emotion, which is mainly caused by autonomic nervous system impairment.</p> Case presentation <p>A patient underwent thoracoscopic segmentectomy for a tumor located in the apicoposterior segment of the left upper lobe. Within two hours after surgery, the patient developed ipsilateral facial pallor accompanied by contralateral flushing, without any neurological deficits or hemodynamic instability. The symptoms persisted until late at night, and facial erythema resolved spontaneously by the following morning. The patient was discharged on postoperative day 4 without complications. Based on the clinical presentation and temporal association, the condition was diagnosed as Harlequin syndrome secondary to paravertebral nerve block at the corresponding vertebral level.</p> Conclusions <p>When performing paravertebral block for pulmonary resection, the location of the tumor and the involvement of the pleura should be considered. If a patient’s preoperative computed tomography (CT) images are consistent with those of the patient described in this article, paravertebral block administration may induce Harlequin syndrome. Therefore, the nerve block procedure may be adjusted to avoid the corresponding segment to mitigate this risk.</p>