The Chiari network, a net-like structure within the right atrium, has been described as an embryonic remnant of no clinical significance. Its role in facilitating paradoxical arterial embolic events and as a potential site for thrombus formation has been reported in the literature. This case illustrates the protective filter-effect of the Chiari network in a 92-year-old women who presented with the clinical signs and symptoms of acute cor pulmonale. Transthoracic echocardiography revealed a large, S-shaped, and extremely mobile right atrial thrombus within the Chiari network. Here, the authors describe how the Chiari network may have prevented the occurrence of a fatal pulmonary embolism acting as a protective factor.