A pericardial cyst developed in a 2-year-old male neutered Maine Coon cat following surgery for an incidentally diagnosed congenital peritoneopericardial diaphragmatic hernia. The cyst caused no clinical signs in the cat, although clinical findings included positional right-sided cardiac tamponade and compression of thoracic structures, associated with a cardiac arrhythmia and axis deviation on electrocardiography. Extensive assessment of the cyst included radiography, echocardiography, computed tomography, exploratory thoracotomy, electrocardiography, histopathology and fluid analysis. Surgical removal of the cyst was curative, and the arrhythmia and axis deviation resolved. This report details case management from initial diagnosis to long-term follow-up, adding to the limited body of literature available on feline pericardial cysts. This is also the first report to associate cardiac arrhythmia with a pericardial cyst.