An 18-year-old woman presented to our institution with fever, bilateral flank pain, headache and photophobia. She had a previous atrial septal defect (ASD) closure device inserted at the age of 9 years. Blood cultures on admission were positive for Corynebacterium diphtheriae, and transoesophageal echocardiogram (TOE) revealed an echodensity associated with the ASD closure device, most consistent with a vegetation. She was treated for infective endocarditis with 6 weeks of intravenous benzylpenicillin, and follow-up TOE showed resolution of the echodensity. To our knowledge, no cases of C. diphtheriaeendocarditis of an ASD closure device have previously been reported.