A 7-month-old boy presented to the hospital with fever, vomiting, and appetite loss and was admitted with a diagnosis of dehydration due to gastroenteritis. Although he stopped drinking and eating, he continued to vomit. The day after admission, systolic murmur, tachypnea, and retractive breathing appeared, and echocardiography showed mitral regurgitation resulting from chordal rupture. Although the vomiting and pleural effusion were improved with medical therapy, retractive breathing persisted. Mitral valve plastic surgery was performed. Mitral valve chordal rupture in infants is rare and presents with nonspecific symptoms. Because most patients need surgical treatment, early and appropriate diagnosis is necessary.