To close the atrial septal defect (ASD) with the transcatheter method, correctly defining the defect and selecting the appropriate closure device size are the most critical steps for the procedure's success. Although ASD can be successfully closed under the guidance of three-dimensional (3D) transesophageal echocardiography (TEE) and two-dimensional (2D) TEE, measurement comparisons between different types of defects are still needed. Our study was designed retrospectively. Sixty-one patients who underwent transcatheter ASD closure with 2D TEE and 3D TEE between 2020 and 2024 were included. The patients were divided into three groups according to the defect shape: circular, oval, and complex; and the measurement results, perioperative process, and clinical outcomes were compared in each group. The average age of the patients was 35.05 ± 13.87 years, and 41 (67.2%) were women. The average follow-up period of the patients was 15.3 ± 9.18 months. No statistical significance was observed in the comparison of measurements obtained with 3D TEE and 2D TEE in the circular and oval defect groups. The differences between the minimum defect diameters of complex defects measured by 2D TEE and 3D TEE (p: 0.037), IVC rims (p < 0.001), aortic rims (p: 0.012), and the differences between implanted device dimensions and the maximum defect diameters measured by both methods were compared; statistical significance was observed (p: 0.025). In circular and non-complex oval defects, it has been observed that the size of the closure device selected with 2D TEE or 3D TEE is optimal, and the procedure is practical and feasible. While the closure of complex ASDs with 3D TEE provides reliable and optimal results, using only 2D TEE in complex ASDs may lead to selecting a smaller-sized device.