There are limited reports of patients with prosthetic valve infective endocarditis (IE) or recurrent IE due to highly beta-lactam-resistant viridans group streptococci. We present a case in which a patient with native valve IE due to beta-lactam-susceptible Streptococcus oralis developed prosthetic valve IE due to highly beta-lactam-resistant S. oralis . A 79-year-old man with a history of native aortic valve IE caused by beta-lactam-susceptible S. oralis 21 months prior to admission and aortic valve replacement was admitted to our hospital with a 2-week history of general malaise and low-grade fever. Transesophageal echocardiography showed a 20 mm vegetation on the prosthetic aortic valve, and emergency cardiovascular surgery was performed on admission day 2. Three sets of blood cultures on admission were positive for highly beta-lactam-resistant S. oralis . Vancomycin and cefazolin were administered as initial treatment. After the surgery, the patient was given vancomycin and gentamicin for 2 weeks, followed by vancomycin for 4 weeks. He was relapse-free at the 6-month follow-up. For patients with native valve IE due to S. oralis who have undergone valve replacement more than 1 year earlier, given the possibility of methicillin-resistant Staphylococcus aureus as well as S. oralis resistance to beta-lactams, it may be advisable to start vancomycin as an initial treatment and continue it until the infecting micro-organism has been proven to be susceptible to beta-lactams.