Purpose: Mirabegron (beta 3 adrenoreceptor agonist) is a new molecule with a mechanism of action distinct from antimuscarinics. Combination therapy with solifenacin was recently studied in an adult population. We evaluated the efficacy and safety of mirabegron as add-on therapy to treat urinary incontinence in children with idiopathic overactive bladder refractory/intolerant to antimuscarinics. Materials and Methods: A prospective off-label study using add-on regimens of mirabegron was conducted in pediatric patients presenting with no symptom improvement while undergoing intensive behavioral and medical therapies and/or significant side effects while undergoing antimuscarinic dose escalation. Our primary outcome was better reported efficacy than with the use of prior antimuscarinic monotherapy. Secondary end points were tolerability, safety and satisfaction. Efficacy and tolerability were assessed based on voiding diaries, post-void residuals, urine cultures, electrocardiograms and vital signs. Families were questioned regarding continence, side effects and compliance. Wilcoxon signed-rank test was used for statistical analysis. Results: A total of 35 patients were recruited at a median age of 10.3 years and were administered add-on mirabegron for a median of 16.4 months. Median bladder capacity improved from 50% to 74% expected bladder capacity (p < 0.001). Continence improved in all patients, with 12 being completely dry. Post-void residual was increased in 2 patients and 1 urinary tract infection was reported. Seven patients reported mild or moderate side effects, with 2 withdrawals because of side effects (1 patient) and post-void residual (1). Conclusions: Add-on mirabegron appears to be a safe alternative for children with refractory overactive bladder. Dual therapy is well tolerated and adjusted dose regimen appears safe in this first pediatric study.