Stomatococcus mucilaginosus is a gram-positive coccus present in the normal flora of the mouth and upper respiratory tract of humans. Although traditionally believed to be an organism of low virulence, S. mucilaginosus has recently been recognized as an emerging opportunistic pathogen, especially in patients with chronic immunosuppressive diseases. This report describes the case of a patient with known mitral valve prolapse, who presented with a mycotic cerebral aneurysm revealing a non-nosocomial spontaneous S. mucilaginosus endocarditis. The spectrum of infections due to this opportunistic pathogen is reviewed, with particular focus on assessing the clinical characteristics and prognosis of S. mucilaginosus infective endocarditis.