Renal cell carcinoma (RCC) can invade locally through the renal vein and into the inferior vena cava (IVC) with tumor-thrombus formation reported in 5%-15% of patients. From the IVC, RCC can grow intravascularly and extend into the right atrium. We present a rare case with two uncommon findings: tumor-thrombus extension leading to a right atrial mass and initial presentation of RCC as heart failure. A 69-year-old woman presented with signs and symptoms of heart failure. Electrocardiogram was normal and the initial troponin level was mildly elevated to 0.09 ng/mL. Echocardiography revealed a dilated right atrium with a 6.9 cm x 3.8 cm echogenic mass consistent with a tumor impinging on the tricuspid valve leading to a functional stenosis. Computed tomography (CT) of the abdomen revealed a large right-sided renal mass with enlargement of the renal vein suggestive of tumor thrombus. Although the initial presentation of RCC with cardiac symptoms is surprising, this case highlights the importance of maintaining a comprehensive differential diagnosis. It also signifies the need for further imaging as not all atrial masses are cardiac tumors. Many other primary tumors - kidney, liver, lung, and thyroid - can directly invade or metastasize into the atrium by way of the vena cava.