Background: Cor triatriatum dexter (CTD) is a congenital anomaly in which the right atrium is divided into 2 parts by a membrane or fibromuscular band. This condition is extremely rare: in high-volume echocardiographic laboratories, the prevalence of CTD is less than 0.01% (1). Cor triatriatum dexter is often associated with other congenital anomalies as atrial septal defect (ASD) (2,3). When the partition is incomplete, it can be asymptomatic and might be detected incidentally by echocardiographic examination or other imaging diagnostic tests (4). Case Summary: In December 2022, a 50-year-old man, symptomatic for palpitation and dyspnea (EHRA II) with paroxystical atrial fibrillation, was referred to our institution and scheduled for transcatheter ablation. He had no signs of chronic heart failure and the laboratory tests were normal. At the physical examination a continuous murmur was heard. We performed a transthoracic echocardiogram (TTE) showing an ASD and a membrane that incompletely divided the right atrium into 2 chambers. CTD or a giant Eustachian valve was suspected. The pre-procedural 3D- transesophageal echocardiography (TEE) showed an incomplete non-obstructive membrane separating the right atrium into upper and lower chambers, thus confirming a diagnosis of CTD (Figure 1, Figure 3). We confirmed an ostium secundum ASD (Figure 2, Figure 3) with an eccentric flow into the right atrium, 0.7 cm2 of area and a not significant left to right shunt (Qp/Qs 0.9). Since the right ventricle function, dimension and pulmonary pressures were normal we didn't address the patient to percutaneous closure. Moreover, we detected a left atrial appendage mobile thrombus formation with LAA reduced velocity and LA smock effect, so the transcatheter atrial fibrillation ablation was postponed. The patient was addressed to rate control strategy and echocardiography follow-up monitoring right ventricle function at 3 months. Conclusions: This case highlights the association between CTD and other congenital cardiac anomalies, including ASD as found in this patient. While cases of cor-triatriatum sinistrum often require correction in infancy, CTD is sometimes diagnosed in adulthood due to the lack of left heart and pulmonary involvement. This report attempts to contribute to what is known about this exceedingly rare disorder, including the possible association with pro-thrombotic status and supraventricular arrhythmias.