An 81-year-old female with a CHA2DS2-VASc score of 6 was admitted to this institution for elective insertion of a left atrial appendage occlusion device (Watchman device). The procedure was successful, with a 21-mm Watchman device inserted into the left atrial appendage after trans-septal puncture under fluoroscopic and transoesophageal echocardiographic (TOE) guidance. The device was stable with tug testing, and imaging revealed adequate device compression. The intraprocedural TOE showed Type IV appendage function with a peak appendage velocity of 40 cm/second, and no spontaneous echocardiographic contrast or thrombus. Day 1 transthoracic echocardiography (TTE) was unremarkable. The patient was discharged on protocol aspirin and clopidogrel for 6 months. One-month postoperatively the patient underwent TTE and TOE studies, which showed a 0.8 x 1 cm newly formed thrombus in the left atrial appendage trapped by the Watchman device. The patient had no neurological symptoms on follow-up. Device-related thrombus can occur on the atrial side, necessitating additional antithrombotic therapy in 3% of cases. It is believed that this is the first Australian case report of thrombus trapped within a Watchman device. The thrombus was unlikely to be clinically relevant, as the device was preventing any thromboembolic events in this high-risk patient. Longer term studies are needed to determine the best antiplatelet and anticoagulant regime to prevent any postprocedure thrombus formation.