Background Multiple myxomas are rare and often associated with Carney syndrome. We present a giant left atrial myxoma that passes through a patent foramen ovale (PFO), mimicking biatrial myxoma.Case summary A 46-year-old female was admitted to the hospital with a cardiac neoplasm without symptoms. The physical examination did not reveal any abnormalities in the skin and endocrine system. Transthoracic echocardiography revealed a large, hypoechoic mass attached to the atrial septum, occupying both the left and right atria. This mass appeared as a bilateral atrial mass on echocardiography. Contrast-enhanced echocardiography revealed a slight enhancement in the mass. The patient was scheduled for surgery without any contraindications. During the operation, a large jelly-like mass originating from the left atrial side of the fossa ovale was found, extending to the right atrium through a PFO. The entire mass was successfully removed, and the atrial septum was repaired. Histopathology confirmed the diagnosis of cardiac myxoma. At the 3-month follow-up, no cardiac abnormalities were observed.Discussion Multiple myxomas have been reported in certain cases of Carney syndrome. A single left atrial myxoma that passes through an atrial septal defect or a PFO can sometimes be misdiagnosed as a bilateral atrial myxoma. Echocardiography plays a crucial role in providing diagnostic information by accurately identifying the location of the myxoma pedicle.