Aim: To systematically review international literature related to rapid maxillary expansion (RME) as the treatment for obstructive sleep apnea syndrome (OSAS) in children less than 18 years-old, followed by a meta-analysis of the apnea-hypopnea index (AHI) before and after RME, with or without a previous adenotonsillectomy (AT). Methods: Literature on databases from PubMed, Wiley online library, Cochrane Clinical Trials Register, Springer link, and Science Direct were analyzed up to March 2020. Two independent reviewers (S.G. and R.J.M.) screened, assessed, and extracted the quality of the publications. A meta-analysis was performed to compare AHI values before and after the treatment with RME. Results: Six studies reported outcomes for 102 children with a narrow maxillary arch suffering from OSAS with a mean age of 6.7 ± 1.3. AHI improved from a M ± SD of 7.5 ± 3.2/h to 2.5 ± 2.6/h. A higher AHI change in patients with no tonsils (83.4%) and small tonsils (97.7%) was detected when compared to children with large tonsils (56.4%). Data was analyzed based on a follow-up duration of ≤3 year in 79 children and >3 years in 23 children. Conclusion: Reduction in the AHI was detected in all 102 children with OSAS that underwent RME treatment, with or without an adenotonsillectomy. Additionally, a larger reduction in the AHI was observed in children with small tonsils or no tonsils. A general improvement on the daytime and nighttime symptoms of OSAS after RME therapy was noted in all the studies, demonstrating the efficacy of this therapy.