Chylothorax is a rare disease of small animals that involves the accumulation of chyle within the pleural cavity. Although chylothorax may be secondary to disease processes such as right-sided heart failure, thoracic duct trauma, or thoracic neoplasia, it is most often idiopathic in nature. The etiology of idiopathic chylothorax is unknown, but is thought to involve lymphangiectasia of the thoracic duct, resulting in the leakage of chyle into the thoracic cavity. This case study details an example of chylothorax in a five-year-old Scottish Terrier, outlining the typical clinical signs, diagnosis, treatment, and prognosis for the disease.