Endocarditis (IE) is defined by an infection of a native or prosthetic heart valve, the mural endocardium, or an indwelling cardiac device. Although viridan-group streptococci (VGS) and Staphylococci species have collectively been considered as the most common cause of endocarditis, uncommon pathogens may also lead to the disease with significant morbidity and mortality. Abiotrophia defectiva, a nutritionally variant streptococci (NVS), is a virulent bacterium that preferentially affects endovascular structure and is implicated in many culture-negative endocarditis with dreadful complications such as heart failure, septic embolization, and valve destruction. Here, we report a case of a 60-year-old male patient, with a past medical history significant for hypertrophic obstructive cardiomyopathy, who was incidentally found to have mitral valve vegetative mass with an uncommon agent, A. defectiva. The patient was successfully treated with antimicrobial therapy. The objective of this article is to describe the possibility of uncommon cause of common diseases and raises awareness of infective endocarditis caused by A. defectiva among clinicians and microbiologists. Early and proper identification of this pathogen is important to achieve a better outcome.