We report two cases of acquired factor XIII deficiency with bleeding events during veno-venous extracorporeal membrane oxygenation (ECMO). Case 1: A 76-year-old man diagnosed with aspiration pneumonia after near-drowning was started on ECMO. Later, the patient presented with hemoptysis and anemia. Blood tests showed a decreased factor XIII activity of 29%. Although the patient recovered after receiving 1200 International Units of factor XIII concentrate, the patient had another episode of decreased factor XIII activity and bloody stool and was treated again with factor XIII concentrate. Case 2: A 48-year-old female diagnosed with pneumonia was started on ECMO. Soon after, she presented with hemoptysis and anemia. Blood tests showed a decreased factor XIII activity of 39%. The patient was treated with 720 IU of factor XIII concentrate with good recovery. Acquired factor XIII deficiency cannot be detected by routine coagulation tests, therefore it may be under-diagnosed in the ICU. Detection of acquired factor XIII deficiency is essential when treating a bleeding ECMO patient.