A five-year-old domestic longhaired cat was evaluated for a seven-day history of worsening respiratory distress. Serum analysis for feline leukaemia virus antigen was positive. Pleural effusion was detected on thoracic radiographs and echocardiography revealed a pericardial effusion and cardiac tamponade. Cytological evaluation of the pleural and pericardial effusions showed lymphoblastic cells indicative of disseminated lymphoma. Following thoracocentesis and pericardiocentesis, the cat was treated for lymphoma using the University of Wisconsin-Madison chemotherapy protocol. The cat was sent home after three days and, at the time of writing (six months after initial presentation), was still symptom free. To the authors' knowledge, this is the first report confirming pericardial effusion and cardiac tamponade in the cat as a direct result of an extranodal lymphoma with cytological evidence of neoplastic cells in the pericardial fluid.