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Transverse sinus (TS) is a normal structure of pericardium, usually visualized on transesophageal echocardiography (TEE). Rarely, when it is prominent, it may be visualized on transthoracic echocardiography (TTE), as a mobile circumscribed mass in the parasternal short-axis view. Due to its location adjacent to left atrial appendage (LAA), it can be misdiagnosed as an LAA thrombus. A 41-year-old woman presented with heart failure signs. TTE showed a circumscribed mass (1 cm × 2 cm), in or adjacent to the LAA, suggestive of an LAA thrombus. Additional maneuvers with the transducer made it obvious that it was a prominent TS, adjacent to the LAA, while the appendage was empty. TEE confirmed the diagnosis.