Heparin-induced thrombocytopenia (HIT) is a rare but serious complication of heparin therapy. It is usually seen between the 5th and 14th days of heparin use and if it is not treated, it is a very high mortality picture. When heparin-induced thrombocytopenia develops, heparin treatment should be discontinued and direct thrombin inhibitors (DTI) should be started. In this article, we presented the diagnosis and treatment process with Fondaparinux in three patients with COVID-19 pneumonia who developed HIT after low molecular weight heparin prophylaxis.