The aim of the present study was to assess the efficacy of intravenous (IV) magnesium sulfate (MgSO4) for children with bronchiolitis. A retrospective cohort study was performed at a pediatric emergency department. Aged between 1 and 24 months, children with moderate/severe bronchiolitis according to the Modified Respiratory Distress Assessment Instrument (mRDAI) score were included. Patients who received 40 mg/kg/dose of IV MgSO4 (group 1, n: 74) or not (group 2, n: 33) were compared. Respiratory rate and mRDAI score significantly decreased at the second hour of MgSO4 treatment and the decrease was observed for 4th, 8th, and 12th hours, compared with group 2. Patients in group 2 had a higher rate of requirement and an earlier start high-flow nasal cannula oxygen therapy and a longer hospital stay than group 1. Intravenous MgSO4 provided significant improvement on clinical severity, need for respiratory support, length of hospital stay, and outcomes.