A 70-year-old female patient was admitted to our department with symptoms and signs of acute heart failure and near syncope. After hospitalization, both transthoracic echocardiography (TTE) and subsequent transesophageal echocardiography (TEE) demonstrated a giant (4.9 x 3.9 cm) mobile, irregular, bright left atrial mass consistent with left atrial ball thrombus (LABT). The mass was found to occlude the left ventricular inflow tract (LVIT) above the mitral orifice (supravalvular) in the presence of normal mitral leaflets. After emergent surgical excision, the pathology of the left atrial mass was found to be consistent with thrombus. The case presented here suffered acute diastolic heart failure and near-syncope due to obstruction of the LVIT above the mitral orifice by a giant LABT.