Background High levels of PM2.5 in Bangkok are concerned. We aimed to study the association of PM2.5 level with pediatric patient visits for respiratory diseases. Methods This retrospective study collected hospital visits of children less than 18 years for respiratory diseases and PM2.5 level during 2018–2020. Number of hospital visits with diagnoses of upper respiratory infections (URIs), acute bronchitis, acute bronchiolitis, pneumonia and asthmatic exacerbation were collected. We analyzed the association of weekly average of PM2.5 level with pediatric patient visits for respiratory diseases using a time-series analysis and a negative binomial model. We adjusted year, seasons, number of influenza and respiratory syncytial virus (RSV) cases as confounders. Results We found a significant association between weekly average of PM2.5 level and patient visits for URIs and acute bronchitis in children less than 2 years and asthmatic exacerbation in children less than 5 years. During winter, when weekly average of PM2.5 level was above 55 µg/m3, there was 40% increase in URIs cases and 80% increase in acute bronchitis cases in children less than 2 years. Conclusions High level of PM2.5 was associated with number of pediatric outpatients with URI, acute bronchitis and asthmatic exacerbation. Younger age group had more significant association.