Objectives: We aimed to evaluate the effect of percutaneous atrial septal defect (ASD) closure in children using right heart indices and serum galectin-3 (Gal-3), as potential biomarkers of right heart remodeling. Methods: This case-control prospective study included 40 children with ASD and 25 control subjects. An echocardiographic evaluation was performed before the procedure, as well as 24 h, 1 month, and 6 months after intervention. Serum Gal-3 was measured before, and 1 month after the procedure. Results: Serum Gal-3 concentration, right atrial (RA) dimensions, right ventricular (RV) dimensions, indexed RA area, and right index of myocardial performance (RIMP) were significantly increased in children with ASD compared with control subjects while tricuspid annular plane systolic excursion (TAPSE) was significantly decreased. Six months after closure, RA, and RV dimensions significantly decreased and RV function improved (RIMP decreased and TAPSE increased). Gal-3 oncentration significantly decreased 1 month after ASD closure, but it did not reach normal range compared with control subjects. A positive correlation between Gal-3 and age at closure, RA area, RV dimensions, and RIMP was observed. A positive correlation was observed between the decrease in Gal-3 concentration and the decrease in RA area and RV dimensions 1 month after ASD closure. A significant negative correlation was observed between TAPSE and Gal-3 concentration before and after intervention. Conclusions: Percutaneous ASD closure can improve right-sided indices and decrease serum Gal-3 concentration. Gal-3 can be used as a sensitive biomarker of right heart remodeling, with a decrease in Gal-3 concentration suggesting reversal of maladaptive remodeling.