A six-year-old male crossbred dog was presented with clinical signs of right-sided heart failure. Echocardiography demonstrated a pericardial effusion with cardiac tamponade, while pericardiocentesis and cytology did not reveal any evidence of malignancy. Pericardial drainage was performed twice over a period of three months to resolve haemodynamic impairment before a subtotal pericardiectomy was performed. Biopsy of parietal and visceral pericardium confirmed the diagnosis of pericardial mesothelioma. Intrathoracic cisplatin combined with intravenous doxorubicin were administered, although neutropenia, mild azotaemia and alopecia were noted as adverse reactions to these drugs. Intravenous cisplatin was repeated 45 days later after the signs of nephrotoxicity had resolved. The dog was still free of disease after 27 months. Intrathoracic chemotherapy after pericardiectomy and early diagnosis are recommended to improve prognosis, having achieved long-term survival in the present case.