Dirofilaria immitis is an emerging parasitic threat in Central Europe with veterinary and zoonotic relevance. The case is described of a fatal autochthonous case in a seven-year-old Cavalier King Charles Spaniel from south-eastern Poland, with no travel history. The dog presented with dyspnea, ascites, cough, syncope, and cyanosis; echocardiography revealed right heart enlargement, tricuspid regurgitation, pulmonary hypertension, and left ventricular diastolic collapse. Euthanasia was performed due to poor prognosis. Necropsy identified six pre-adult worms in the right atrium, ventricle, and pulmonary outflow tract, with pulmonary congestion and oedema. This prepatent infection confirms local transmission of D. immitis in Poland and highlights the growing risk of autochthonous dirofilariosis in Central Europe, driven by climate change, vector spread, and animal movement. Vigilance is warranted given the public health implications and potential for misdiagnosis of human pulmonary lesions.