A 4-year-old, European, male neutered cat was admitted to the emergency service showing symptoms of dyspnoea and lethargy. Radiographs and echocardiography confirmed a pericardial effusion. Fluid analysis obtained via thoracocentesis showed evidence of bacteria in the pericardial space. The species of bacteria was Neisseria animaloris. Subtotal pericardiectomy through sternotomy was the treatment of choice in this case to perform a surgical lavage and the placement of a thoracic drain in order to decrease the pericardial pressure. Sterile samples were taken during the surgery. The cat was discharged 6 days after admission, with oral antibiotic treatment (amoxicillin/clavulanic acid) and was free of clinical signs at discharge.