Myocardial contusion is the most common cardiac injury from blunt chest trauma (BCT), whereas isolated valve injury is uncommon. We report a case of acute mitral regurgitation (MR) due to isolated valve injury after BCT. A 60-year-old man received an impact on his left chest by a car wheel three weeks prior to visiting our hospital. At the time a diagnosis of contusion of the chest wall without rib and sternal fractures was made. Thereafter, the patient had progressive worsening of heart failure symptoms. Eventually he developed dyspnea on slight exertion but echocardiographic evaluation was not performed at the time of diagnosis or during the three weeks prior to admission. At admission a holosystolic murmur was heard. Transthoracic echocardiography revealed prolapse of the posterior mitral leaflet due to torn chordae tendineae with severe MR and normal left ventricular wall motion. At surgery, torn chordae tendineae and a leaflet tear of the posterior leaflet were detected, and mitral valve repair was achieved without residual MR. Pathological examination of the torn chordae showed no findings of endocarditis or myxomatous degeneration. Echocardiography may play an important role for accurate and prompt diagnosis of cardiac lesions in patients with recent or a history of high-energy BCT.