A 56-year-old woman with rheumatoid factor-positive rheumatoid arthritis underwent FDG-PET/CT because of fatigue, fever, coughing, and weight loss for several months. FDG-PET/CT solely revealed a mildly hypermetabolic hypodense area in the left atrium. Subsequently, transthoracic echocardiography and contrast-enhanced MRI showed a left atrial pedunculated soft tissue mass suggestive for myxoma, with histological confirmation. Myocardial involvement by tumors is rare, and FDG-PET/CT has been very useful for identifying cardiac metastases. However, very few cases have been reported using FDG-PET/CT for detecting primary cardiac tumors, but as shown here, abnormal focal myocardial uptake should trigger further morphological assessment.