We present a case report of a 28-year-old man with a huge tumor that occupies both dilated atria almost completely but not extending into the ventricles. Cardiac rhabdomyosarcoma (CR) is extremely rare in adult patients. Its rarity and heterogenous clinical presentation make diagnosis difficult. In our case, after we have made an initial diagnosis via echocardiography, we took biopsies under computed tomography (CT) guiding. Histologic examination showed a CR. We want to emphasize that CT-guided biopsy is a useful and applicable technique in establishing preoperative diagnosis of intracardiac masses.