Thymic cysts (TCs) represent 1-2% of all mediastinal masses. To the best of our knowledge transudative effusion due to rupture of a TC into the pleural cavity, as it happened with our patient, has never been described before. The patient was admitted in 5th Department of Respiratory Medicine of "SOTIRIA" Hospital complaining of pleuritic chest pain in the right hemithorax and dyspnea on exertion. Clinical and laboratory examinations indicated a right pleural effusion. Then, the patient was transferred to the 2nd Department of Propedeutic Surgery of 'LAIKO' Hospital where he underwent surgery. Video-assisted thoracic surgery (VATS) revealed an enormous 25-cm cyst ruptured into the right pleural cavity. The cyst was removed by open thoracotomy due to adhesion to contiguous tissues. Pathological examination indicated thymic origins.