We report on a 10-year-old female patient who rapidly developed a left atrial (LA) mass 2 months after orthotopic heart transplant. Nine days prior to detection of the mass, she received high-dose corticosteroids for acute cellular rejection (grade 2). Despite negative echocardiogram 5 days prior to detection, a large echogenic mass was noted in the LA (18 x 12 x 24 mm); it was surgically resected after unsuccessful anticoagulation treatment. Pathogenesis of this LA thrombus remains uncertain, but immunosuppression, acute rejection, and high-dose steroid therapy may have contributed. Surgical thrombectomy is a safe and effective treatment option for LA thrombus.