Introduction Postoperative neck hematoma (PNH) is an uncommon but potentially-lethal complication of thyroid surgery. Objective To identify the risk factors for postthyroidectomy hematoma requiring reoperation, the timing, the source of the bleeding, the related respiratory distress requiring tracheotomy, and the late outcomes. Methods We retrospectively analyzed the records of 5,900 consecutive patients submitted to surgery for thyroid diseases at a single institution. Results In total, PNH occurred in 62 (1.1%) patients. Most cases of bleeding occurred within the first 6 hours after thyroidectomy, but 12.5% of the hematomas were observed after 24 hours. Obvious bleeding points were detected in 58.1% of the patients during the reoperation, with inferior thyroid artery branches and superior thyroid vessels being the most frequent bleeding sources. Only two patients required urgent tracheostomy. There were no hematoma-related deaths. Permanent hypoparathyroidism and recurrent laryngeal nerve injury are more frequent following reoperation for PNH. The factors significantly associated with PNH were: older age, concurrent lymph node dissection, and chronic lymphocytic thyroiditis. Gender, previous neck irradiation, presentation at diagnosis (symptomatic or incidental), substernal goiter, thyroidectomy for hyperthyroidism, prior thyroid surgery, malignant histology, the extent of the surgery (total versus non-total thyroidectomy), the use of energy-based vessel sealing devices, the use of the hemostatic agent Surgicel, and the placement of a surgical drain were not significantly associated with PNH. Conclusion Hematoma after thyroid surgery is an uncommon complication, but it is related to significant postoperative morbidity. A better understanding of the risk factors and of the time until hematoma formation can help target high-risk patients for preventive measures and closer postoperative observation.