Congenital portosystemic venous connections are rare vascular anomalies that connect the portal and systemic venous circulations. These vascular lesions can lead to complex and varied physiologic manifestations in single-ventricle patients as they progress through the various stages of palliation in the Fontan pathway. Specifically, these connections may be unmasked after a superior cavopulmonary anastomosis operation, but then “re-masked” after Fontan completion. We describe the complex physiology with an illustrative case report and present a novel method to unmask these pathologic connections after Fontan completion to facilitate transcatheter occlusion and resolve downstream symptoms.