Abstract: A three-month-old male uncastrated Maine coon cat was presented for sudden inappetence, fatigue, and respiratory distress. In order to diagnose and treat the cat, hematology, X-ray, and echocardiography were used for preliminary diagnosis, and corresponding treatment was performed according to the diagnosis results. The results showed that according to medical history, clinical symptoms and imaging examinations, the cat had cor triatriatum sinister, secondary congestive heart failure on the left side, right ventricular eccentric hypertrophy and pulmonary hypertension. The preoperative supportive therapy was not effective, and the cat was finally euthanized and necropsy. The results of the necropsy verified the clinical diagnosis and it was found that the cat had horseshoe kidney. The results suggested that echocardiogram was a key tool for diagnosing cor triatriatum sinister in cats.