We herein report the case of a patient with an acute cervicomediastinal hematoma secondary to the spontaneous rupture of a parathyroid adenoma. A 47-year-old female presented with swelling and pain in the neck. She had no history of trauma or of having undergone any medical or odontological procedures. An ultrasound examination revealed the presence of an obscure mass located behind the right lobe of the thyroid gland. A computed tomography scan showed the presence of a low-density lesion extending from the retropharynx to the mediastinum, a high-density lesion located behind the right lobe of the thyroid gland and a right pleural effusion. Because the patient's neck swelling and anemia gradually worsened, she underwent emergency surgery. The neck was found to be swollen due to a hematoma; however, no abscesses were detected in the operative field.Thoracoscopy of the right chest showed no active bleeding. The fragmented mass was histopathologically diagnosed as a parathyroid adenoma with acute hemorrhage, which is quite rare. Our experience suggests that, in patients with severe cervicomediastinal hematomas without any trauma or trigger, a diagnosis of spontaneous rupture of a parathyroid gland lesion should be considered.