Common atrium (CA), also called three-chambered heart or cor triloculare biventriculare, is one of the rare congenital anomalies, (Garg et al., 2020) defined by a complete absence of the atrial septum, eventually associated with malformation of the atrioventricular (AV) valves. We report the case of a 57-year-old woman with CA complicated with Eisenmenger syndrome, who suffered from symptomatic persistent atrial fibrillation (AF). The patient did not have any muscular, skeletal, ophthalmologic, or vascular abnormalities to signify that her abnormalities were part of any congenital syndrome.Visceroatrial, atrioventricular, and ventriculoarterial concordance were noted. Nevertheless, she also presented inferior vena cava (IVC) interruption. During her last clinical evaluation, a mild holosystolic murmur was heard at all auscultation points, gaining intensity at the apex and mesocardiac area. On physical examination, while S1 was normal, a widely split S2 was heard. There was no pretibial edema, and the lungs were clear. Oxygen saturation measured with finger pulse oximetry was 92%.Her last echocardiography evaluation showed in apical view the absence of interatrial septum (Figure 1). CA surface was 50 cm 2 . Left ventricular ejection fraction was 60%. In addition to right ventricular dilatation, moderate tricuspid regurgitation was seen, along with severe pulmonary hypertension. Tricuspid annular plane systolic excursion 17 mm. Mitral and tricuspid valve attachments to the interventricular septum were on the same anatomic plane, and there was a small cleft present in the mitral valve, with mild regurgitation. Paradox movement of the interventricular septum was observed but without any interventricular communication.Cardiac catheterization revealed systolic pulmonary arterial pressure around 102 mmHg. Right ventricular pressure was 116/0 mmHg denoting severe pulmonary hypertension, RV end-diastolic pressure at 21 mmHg, mean CA pressure as 13 mmHg, and left ventricular end-diastolic pressure as 12 mmHg.The patient was treated with furosemide, spironolactone, with anticoagulation by rivaroxaban. Rate control was achieved with a low dose of acebutalol. Because CA was diagnosed at a late stage,