BACKGROUND Giant coronary artery aneurysms (CAA) are extremely rare and can mimic cardiac tumors. Therefore, an unidentified mass in the heart requires a multimodality imaging approach for accurate diagnosis and guidance of further management, which for CAAs often include surgical intervention to prevent complications such as thrombosis or rupture. CASE REPORT A 37-year-old man presented with non-specific symptoms. A CT scan revealed multiple bilateral pulmonary embolisms and an indeterminate mass in the right atrium. Transthoracic echocardiography (TTE) showed a suspected cardiac tumor, and further imaging with transesophageal echocardiography (TEE), magnetic resonance imaging (MRI), and position emission tomography (PET) indicated a local inhomogeneous mass with arterial perfusion. A preoperative cardiac CT found the mass to be a giant thrombosed CAA in the proximal right coronary artery compressing the tricuspid annulus. The patient underwent successful surgical excision of the CAA along with coronary artery bypass grafting. Postoperative management included lifelong administration of acetylsalicylic acid and a 3-month course of anticoagulant therapy. Histopathology excluded systemic vasculitis, indicating a congenital etiology for the CAA. CONCLUSIONS This case illustrates the indispensable role of coronary CT angiography in accurately diagnosing and managing complex cardiac conditions. Due to the complex and diverse nature of suspected cardiac tumors, cardiac CT should always be added in the diagnostic workup to describe the coronary anatomy in relation to the tumor and to identify a differential diagnosis such as a giant coronary aneurysm.