A 69-year-old woman with a history of diabetes mellitus presented at our emergency room with chest pain and dyspnea. A chest computed tomography revealed a pericardial effusion. Pericardiocentesis was performed; strains of the Streptococcus milleri group were detected on culture of the fluid thus obtained. Therefore, purulent pericarditis was diagnosed. Despite treatment with panipenem/betamipron, the pericarditis worsened leading to the development of cardiac tamponade. Emergency pericardial drainage was performed, after which the condition resolved without any complications. We report an extremely rare case of purulent pericarditis caused by a strain of the Streptococcus milleri group. In addition, we review 5 previously reported cases of purulent pericarditis caused by strains.