Purulent pericarditis caused by Morganella morganii is extremely rare. We report herein a case of a 61-year-old man who presented with chest pain and dyspnea fourteen days after chemotherapy for multiple myeloma. Echocardiogram and computed tomography revealed a massive pericardial effusion and associated cardiac tamponade. Pericardiocentesis was performed. Pericardial fluid was found to be purulent, and Morganella morganii was isolated from the fluid. The patient was successfully treated with antibiotic therapy and surgical drainage of the fluid. Morganella morganii should be considered a possible pathogen when immunocompromised patients develop purulent pericarditis.