This report presents a case of life-threatening iliopsoas haematoma in an immunosuppressed 86-year-old man with a history of prostate cancer during the clinical course of coronavirus disease 2019 (COVID-19). The patient was hospitalized for COVID-19-associated pneumonia. One week after admission, he complained of pain in his right thigh when he changed his position. Laboratory findings revealed markedly progressive anaemia and elevated creatine phosphokinase levels. Chest computed tomography revealed a massive haematoma in the right iliopsoas muscle spreading to the retroperitoneal space. Considering the advanced age and status of the patient, he was treated with red blood cell transfusions and bed rest. Fortunately, the anaemia was improved, and the haematoma gradually reduced in size. It should be noted that even during isolation, careful physical examination is important. In addition, physicians who administer heparin to patients with COVID-19, even if prophylactic, should be aware of bleeding complications.