Background: Respiratory syncytial virus (RSV) is an important cause of bronchiolitis and pneumonia in children globally. This study aimed to incorporate new data to update estimates of RSV burden in children through 5 years of age in Western Pacific and Southeast Asia Regions. Methods: A systematic review and meta-analysis was conducted to examine the proportion of RSV among cases of respiratory tract infection (RTI) in children in Western Pacific and Southeast Asia Regions using random effects models. Studies were eligible if they met the following inclusion criteria: (1) observational studies such as cohort and cross-sectional studies; (2) studies on humans; (3) studies on patients with respiratory tract infection (RTI) or influenza-like illness (ILI); (4) studies reporting incidence or proportion of RSV infection among respiratory related illness; and (5) studies on children aged 5 years or less. Findings: A total of 4,403 studies were identified from an initial search. After screening titles, abstracts, and full-text review, a total of 173 studies that met predefined eligibility criteria were included in the analysis. The overall proportion of RSV infections among all ARTIs was 18.7% (95% CI: 16.0-21.5%), while the proportion of RSV infections among LRTIs was 28.7% (95% CI: 2.6-30.3) in children in Western Pacific and Southeast Asia Regions between 1970 and 2021. The proportion trend increased from 10.6% (95% CI: 2.90-22.2%) in the 1970s to 26.8% (95% CI: 22.0-31.9%) in the 1980s and 33.3% (95% CI :14.7-55.3%) in the 1990s. It then decreased to 25.0% (95% CI 22.8-27.3%) in the 2000s and 19.7% (95% CI 17.3-22.2%) in the 2010s. By country, Myanmar (50.0%; 95% CI, 47.5-52.4%) and New Zealand (45.3%; 95% CI, 37.2-56.4%) had the highest proportion during the overall time period, followed by Bhutan (45.2%; 95% CI, 36.4-54.3%), Lao PDR (41.0%; 95% CI, 36.2-46.0%), and Vietnam (36.0%; 95% CI, 19.3-53.6%). Interpretation: Substantial RSV-associated disease burden occurs in children in Western Pacific and Southeast Asia Regions. Our findings provide new and important evidence of the need for RSV prevention in Western Pacific and Southeast Asia countries. They could inform future preventive policy.