We herein report a 56-year-old man with severe hypocalcemia during ruxolitinib therapy for myelofibrosis transitioning from JAK2 mutation-positive polycythemia vera. Blood transfusions were administered every one to two weeks for ruxolitinib-induced anemia. Blood tests revealed hypocalcemia with low TRACP-5b, 25-hydroxyvitamin D (25 (OH) D), and 1,25-dihydroxyvitamin D₃ (1,25 (OH) ₂ D₃) levels within the lower reference range. Intact-PTH levels were relatively low compared to calcium levels. Severe hypocalcemia with ruxolitinib is rare and may be caused by a combination of factors, impaired vitamin D activation due to liver or renal insufficiency, accumulation of calcium-chelating agents from blood transfusions, and inadequate compensatory response to PTH.