Previous studies on the association between fine particulate matter (PM2.5) exposure and acute respiratory infection in children are scarce and present inconsistent results. We estimated the association between short-term PM2.5 exposure and acute respiratory infection among children aged 0-4 years using a difference-in-differences approach. We used data on the daily PM2.5 concentrations, hospital admissions for acute respiratory infection, and meteorological factors of the 15 regions in the Republic of Korea (2013-2015). To estimate the cumulative effects, we used a difference-in-differences approach generalized to multiple spatial units (regions) and time periods (day) with distributed lag non-linear models. With PM2.5 levels of 20.0 μg/m3 as a reference, PM2.5 levels of 30.0 μg/m3 were positively associated with the risk of acute upper respiratory infection (relative risk (RR) = 1.048, 95% confidence interval (CI): 1.028, 1.069) and bronchitis or bronchiolitis (RR = 1.060, 95% CI: 1.038, 1.082) but not with the risk of acute lower respiratory infection and pneumonia. PM2.5 levels of 40.0 μg/m3 were also positively associated with the risk of acute upper respiratory infection (RR = 1.083, 95% CI: 1.046, 1.122) and bronchitis or bronchiolitis (RR = 1.094, 95% CI: 1.054, 1.136). We found the associations of short-term PM2.5 exposure with acute upper respiratory infection and bronchitis or bronchiolitis among children aged 0-4 years. As causal inference methods can provide more convincing evidence of the effects of PM2.5 levels on respiratory infections, public health policies and guidelines regarding PM2.5 need to be strengthened accordingly.