To investigate the feasibility and accuracy of cardiac multidetector computed tomography (MDCT) prosthesis sizing prior to ostium secundum atrial septal defect (ASD) percutaneous closure. Seventy consecutive patients were included in this retrospective bicentric study between May 2012 and June 2018. All underwent cardiac MDCT (primarily performed to rule out abnormal venous pulmonary return and coronary anomaly) and transesophageal echocardiography (TEE) before transcatheter closure: dimensions of the defect and peripheral rims were measured. Measurements of the defect obtained at TEE and MDCT were compared to prosthesis size. Our primary objective was the comparison of ASD maximal diameter obtained at MDCT (CT-Dmax) to prosthesis size. Intraclass correlation coefficient (ICC), Bland Altman plots and linear regression were calculated. Intra- and inter-observer agreements were calculated for MDCT defect measurements. Forty-three patients were finally included for defect measurements: 17 patients did not undergo transcatheter closure, and 10 had incomplete data. For CT-Dmax, ICC was 0.88 (CI 95% = [0.78-0.93]; p = 0.06); mean difference was - 0.8 ± 5.7 mm; regression linear equation was 0.9 × + 3.2 (p < 0.001). For maximal diameter at TEE versus prosthesis size, ICC was 0.46 (CI 95% = [0.21-0.61]; p = 0.003); mean difference was-6.0 ± 8.2 mm; regression linear equation was 0.91 × + 7.6 (p < 0.001). Intra- and inter-observer agreement for CT-Dmax were 0.97 (CI 95% = [0.95-0.98]) and 0.86 (CI 95% = [0.73-0.93]) respectively. MDCT is a reliable tool for sizing the defect of ostium secundum ASD, making it a complement or even an alternative to pre-procedural TEE.