Synovial sarcoma, which is a soft tissue malignancy, primarily affects the extremities in the para-articular regions in adolescents and young adults. Synovial sarcoma of the pleural cavity is extremely uncommon, and there have been only a few reports in the literature. We report here an unusual case of left-sided spontaneous hemothorax as a presenting manifestation of intrathoracic synovial sarcoma in a 33-year-old pregnant woman. Spontaneous hemothorax, unrelated to trauma, is a very unusual clinical presentation, but we must consider entities with a physical risk assessment promptly because of the possible need for urgent management based on the hemodynamic condition of the patient. We should always be aware of possible causes of spontaneous hemothorax.