Polycythemia vera (PV) is one of myeloproliferative neoplasms characterized by a remarkable increase in myeloid cells caused by abnormality of the hematopoietic stem cells. In patients who receive surgical treatment for PV, conflicting complications such as thrombosis and hemorrhage develop frequently. We report a case of PV diagnosed after postextraction hemorrhage. A 75-year-old woman was found to have polycythemia on a health checkup two years previously, but did not receive any treatment for it. She had her left maxillary wisdom tooth extracted at a dental clinic, but was referred to our hospital immediately to manage postextraction hemorrhage. We performed hemostatic treatment with the patient under local anesthesia. Because of the severe hemorrhage, blood examinations were performed and showed increased counts of white blood cells, red blood cells, and platelets. Therefore, we consulted a hematologist, and she received additional examinations. JAK2 gene mutation was positive, bcr/abl t (9;22) translocation was negative, and decreased erythropoietin was revealed. From these results, she was given a diagnosis of PV according to the WHO diagnostic criteria in 2016. Now, six months after tooth extraction, she is being treated by regular phlebotomy, daily oral aspirin, and hydroxyurea to control the blood cells.