A 16-year-old boy was admitted to a district hospital with fever, headaches, abdominal pain, vomiting, and heart murmur. Thickening of the mitral valve leaflet with regurgitation was diagnosed by a cardiologist, who, suspecting infective endocarditis, referred the patient to a specialist centre. The diagnosis was confirmed by blood cultures showing infection with a non-toxigenic species of Corynebacterium diphtheriae. Antibiotic treatment was continued. The boy was urgently referred to cardiac surgery to remove the affected mitral valve and implant a St. Jude 31 mm mechanical prosthesis. The postoperative course was uncomplicated, and after 8 days he was transferred to the Department of Cardiology, where treatment with antibiotics and anticoagulants was continued, maintaining the international normalised ratio of 2.5–3. He was discharged home in good condition with the recommendation to continue antibiotic therapy for another three weeks, monitor inflammatory markers, as well as continue cardiac medications and anticoagulants. Conclusion – Corynebacterium diphtheriae can cause endocarditis.