We report a rare case of interventricular septal hematoma after patch closure of a perimembranous ventricular septal defect in a 10-month-old infant. Intraoperative transesophageal echocardiography was not performed. Routine transthoracic echocardiography at the 1st postoperative hour showed a huge intramural hematoma causing severe thickening of the ventricular septum. The patient's hemodynamics were stable and surgical revision was not required. The patient recovered well without complication.