Urinothorax is a rare cause of pleural effusion in which urine accumulates within the pleural cavity. Although identified as early as the 1950s, fewer than 100 cases have been reported since then. The causes of urinothorax are generally divided into two categories: urinary obstruction and urinary tract injuries, which include both iatrogenic and traumatic causes. Pleural fluid analysis can aid in diagnosis and typically reveals a hallmark transudative effusion, characterized by a low pH and a pleural fluid-to-serum creatinine ratio greater than 1. Pleural fluid protein is usually low, and lactate dehydrogenase levels can vary depending on the underlying cause. Confirmatory imaging studies, such as CT urography and renal scintigraphy, can reveal urine leaking into the peritoneal or retroperitoneal space. Drainage of pleural fluid can serve both diagnostic and therapeutic purposes based on the symptoms. Complete resolution of urinothorax requires identifying and treating the underlying cause. Although rare, urinothorax should be suspected in patients presenting with a new pleural effusion accompanied by signs of obstructive or traumatic uropathy. In this chapter, we discuss the history, pathophysiology, clinical presentation, diagnostic evaluation, and treatment options for urinothorax.