Primary cardiac sarcomas are very rare. Infiltrative cardiac tumors may be difficult to diagnose by transthoracic echocardiography (TTE) only. Herein, we report a case of primary unclassified cardiac sarcoma with clinical and echocardiographic manifestations of mitral stenosis (MS). The tumor was not identified by TTE preoperatively because of its diffuse infiltration of the left atrial wall and both mitral leaflets without protruding mass, and was only discovered by intraoperative transesophageal echocardiography (TEE). This report alerts clinicians that TEE is a necessary adjunctive tool to facilitate the correct diagnosis in patients with obscure etiologies of mitral valve diseases especially when they will receive surgical intervention.