This case report details the presentation, clinical course, and resolution of a young, intact male Weimaraner dog with history of chronic cough and lethargy, progressing to severe respiratory distress and cardiovascular compromise. Thoracic radiography showed evidence of pericardial effusion and two-dimensional echocardiography demonstrated a large, discrete intrapericardial cyst causing compression of the right side of the heart. The patient underwent surgical excision of the cyst with subtotal pericardectomy, and made a full recovery. Histologic examination of the mass was consistent with a unilocular cystic hematoma and adipose tissue incorporated into the pedicle of attachment. Differential diagnosis, treatment and discussion of possible etiopathogenesis is presented.