Cor triatriatum dexter is a rare congenital abnormality which results in clinical signs similar to a Budd-Chiari type lesion or right sided congestive heart failure. In this case report, a three month old male intact Staffordshire Bull Terrier presented with signs of emaciation and ascites. Cardiac evaluation demonstrated an anomalous membrane separating the right atrium into a cranial chamber and severely dilated caudal chamber. Color flow and continuous wave Doppler echocardiography revealed a high flow velocity and a 9 mmHg pressure gradient across a fenestra in the membrane. These findings are consistent with Cor triatriatum dexter and a balloon septostomy was performed. Although a decrease in pressure gradient across the fenestra was achieved, the procedure was unsuccessful at obtaining complete resolution of clinical signs, thus open heart surgery is currently being contemplated.