Background: COVID-19 is associated with an increased risk of venous thromboembolism (VTE), and prophylactic anticoagulation is recommended for the prevention of VTE in COVID-19 patients. We encountered a patient with COVID-19 who developed iliopsoas hematoma (IPH) that was likely caused by prophylactic anticoagulation against VTE; we report the case here because IPH is an important risk in rehabilitation treatment. Case: The patient was a 73-year-old man with severe COVID-19 who received anticoagulation therapy from the time of admission (day 0). On day 22, decreased hemoglobin levels, muscle weakness in the left lower extremity, and pain on passive movement of the left hip joint were noted. On day 29, computed tomography (CT) was performed and revealed a mass lesion suspicious of a hematoma in the left iliopsoas muscle. On day 36, magnetic resonance imaging (MRI) was carried out to re-evaluate the mass lesion and revealed a multicystic lesion that could also have been an abscess. CT-guided puncture drainage was performed, but no pus-like material was collected; this finding led to a diagnosis of IPH. Subsequent exercise loads were gradually increased while the status of the hematoma was assessed. Discussion: The prevalence of IPH in COVID-19 patients has been reported to be 7.6 cases per 1000 admissions, and the use of anticoagulation is likely to increase the risk of IPH. Because rehabilitative interventions can lead to the discovery or aggravation of IPH, the possibility of IPH should be kept in mind when providing rehabilitation treatment for COVID-19 patients.