Intracardiac thrombus in infants is an exceptionally rare finding and often associated with underlying congenital heart disease, central venous catheters, or coagulation abnormalities. Left atrial thrombi are typically encountered in adults with atrial fibrillation, but their occurrence in infants without structural heart defects is unusual. We report the case of an infant who developed a large left atrial thrombus following sustained supraventricular tachycardia (SVT). Echocardiography revealed dilated atria, atrioventricular valve regurgitation, and a large clot in the left atrial appendage. The child was managed conservatively with anticoagulation, resulting in significant thrombus resolution over 45 days. This case highlights the importance of early echocardiographic evaluation in infants with persistent arrhythmia and demonstrates the efficacy of anticoagulation therapy in resolving thrombi without surgical intervention.