A 15-year-old domestic shorthair cat was presented with a five-day history of progressive inappetence, lethargy and dyspnoea. Thoracic radiographs revealed pleural effusion and suggested pericardial effusion. Echocardiography revealed a pericardial effusion, cardiac tamponade and focal thickening of the pericardium. Diagnostic and therapeutic pericardiocentesis was performed and led to immediate but short-term alleviation of the clinical signs. Both the pleural and pericardial effusion were modified transudates and contained reactive mesothelial cells. Thoracoscopic pericardiectomy was performed which revealed a moderate amount of pleural effusion, and a diffusely thickened and irregular pericardium but no other abnormalities were found. Histopathological examination of the pericardium revealed an aggressive anaplastic carcinoma. Thoracoscopy gave a far superior visualisation of thoracic structures compared to that expected at open thoracotomy. Thoracoscopic pericardiectomy was an efficient procedure with a good postoperative recovery.