Primary cardiac sarcoma is a rare disease in adults. It is also associated with poor prognoses, due to diagnostic delay, therapeutic difficulty, and high metastatic potential. The coincidence of pregnancy and a primary cardiac intimal sarcoma is extremely rare. We report a pregnant woman at 27(+5) weeks gestation who was admitted to the hospital with acute-onset dyspnea. A mass was found on the left atrium by transthoracic echocardiography. Subsequently, the intracardiac mass was removed, and mitral valve replacement and modified DeVega tricuspid annuloplasty were performed. The patient was diagnosed with a undifferentiated sarcoma, and gave birth to a 1,230 g living baby boy by Caesarean section from preterm contraction at 29(+5) weeks gestation. The patient then received systemic chemotherapy. However, 10 months after the initial clinical onset, the patient suddenly died. Surgery is the standard treatment for cardiac tumors, and their removal should always be attempted, even in pregnant women. Although the overall survival rates of the patients are rather poor, palliative cardiac surgery allows the prolonging of pregnancy, until an acceptable fetal viability level is reached.