An 8-month-old boy presents with 1 week of gradually worsening respiratory distress. After chest radiograph shows bilateral airspace opacities, he is treated for pneumonia with antibiotics and intravenous fluids. Shortly after this, he decompensates requiring intubation, followed by cardiopulmonary support with pressors and eventually extracorporeal membrane oxygenation. Bedside ultrasound then shows an additional congenital membrane in the left atrium obstructing pulmonary venous return and thought to have caused his original presentation and subsequent decompensation. After surgical correction, he is decannulated from extracorporeal membrane oxygenation, makes a quick recovery, and is discharged from the hospital in good condition within 2 weeks. This rare case of cor triatriatum highlights the importance of considering congenital heart disease even with common presentations in the pediatric population.