We present a case of prosthetic valve endocarditis, which was caused by a rare etiological agent, Streptococcus constellatus. In our case, transesophageal echocardiography showed a large and broad abscess formation throughout the patient's aortic prosthetic valve ring and left atrium. Despite specific intravenous antibiotic therapy, the infection was uncontrollable, and the patient underwent surgical treatment. The pathogen rarely causes endocarditis, but it is known to have a strong potential to form abscess, and therefore its infection may be more serious than other Streptococcus species. Our case seems to be the first report of surgical treatment of PVE caused by S. constellatus. As in our case, transesophageal echocardiography is useful for the detection of vegetation and abscess, and early cardiac surgery may be a more appropriate therapeutic approach for endocarditis caused by S. constellatus.