Transcatheter atrial septal defect (ASD) closures using an Amplatzer Septal Occluder (ASO) have been widely performed. Compared to children, we sometimes experience late recovery of exercise performance in adult patients. Our study aimed to evaluate the change in the cardiopulmonary exercise capacity in asymptomatic or mildly symptomatic adult patients after a transcatheter ASD closure using an ASO. The subjects consisted of 29 patients (age 39.5 ± 13.6 years) that underwent cardiopulmonary exercise testing (CPX) before, 3, 6, and 12 months after a transcatheter secundum ASD closure using an ASO. The peak oxygen consumption (peak VO₂), anaerobic threshold (AT), and slope of the correlation between the ventilation and carbon dioxide production (VE/VCO₂ slope) were evaluated. We also evaluated the left-ventricular end-diastolic diameter (LVEDD), right-ventricular end-diastolic dimension (RVEDD) by echocardiography, and hemodynamic values by cardiac catheterization before the ASO procedure. The peak VO₂ did not show any improvement 3 months after the ASO procedure; however, a significant improvement was displayed 6 and 12 months (baseline: 23.4 ± 6.3, 3 months: 23.6 ± 6.4, 6 months: 25.1 ± 5.6, 12 months: 26.4 ± 5.3 mL/kg/min; p < 0.001) after the ASO. The LVEDD (before: 38.1 ± 3.6, 3 months: 43.4 ± 3.4 mm; p < 0.001) and RVEDD (before: 33.6 ± 5.3, 3 months: 26.3 ± 2.6 mm; p < 0.001) on echocardiography quickly improved 3 months after the ASO. Although the LVEDD and RVEDD normalized 3 months after the ASO, the peak VO₂ still decreased; however, the peak VO₂ improved to almost a normal range 6 months after the ASO.