Cardiac myxomas are rare in children and can mimic other conditions. We report a 14-year-old boy initially diagnosed with acute poststreptococcal glomerulonephritis based on hematuria, mild proteinuria, and elevated anti-streptolysin-O titers. However, worsening dyspnea, edema, and signs of heart failure prompted further evaluation. Echocardiography revealed a large left atrial mass with impaired biventricular function and tricuspid regurgitation. Surgical excision confirmed a myxoma. The patient's renal and systemic symptoms were attributed to low cardiac output and venous congestion, not true glomerulonephritis. This case highlights the need to consider cardiac causes in pediatric patients with persistent nephritic-like features. © 2026 Annals of Pediatric Cardiology.