An 8-month-old female British shorthair cat presented for a routine health screening. The radiography examination showed an unusual cardiac silhouette around the cardiac apex. The ECG segment from S wave to the T wave (ST segment) is elevated and reveals a counterclockwise deviation of the cardiac axis. An echocardiographic examination demonstrated the loss of the pericardium over the right area of the heart. After a final diagnosis of partial absence of the pericardium, diaphragmatic plication was performed to prevent diaphragmatic herniation. This surgical technique can be selected as a treatment choice to prevent and repair the diaphragm. The partial loss of the pericardium is a rare congenital malformation that rarely occurs in veterinary medicine. This is the first report describing the diagnosis and prevention of a partial loss of the pericardium in cats.