Short-term exposure to ambient air pollution is associated with risk of asthma attacks. We investigated the association between ultra-short-term exposure to air pollution and risk of childhood asthma exacerbations. Hourly data on emergency department visits (EDVs) for asthma in children during 2013–2015 in Brisbane, Australia, were obtained. We undertook time-stratified case-crossover analyses to examine the hourly association between exposure to air pollutants (particles with diameter ≤10 μm (PM10), particles with diameter ≤2.5 μm (PM2.5), ozone (O3), and nitrogen dioxide (NO2)) and risk of EDVs for asthma after controlling for temperature, relative humidity, public holidays and circulating levels of influenza and respiratory syncytial virus. Risk of EDVs for asthma increased within a few hours after exposure to O3 (odds ratio [OR]: 1.170, 95% confidence interval (CI): 1.003–1.365) or NO2 (OR: 1.359, 95%CI: 1.049–1.760). The association between O3 exposure and risk of EDVs for asthma was stronger in boys (OR: 1.244, 95%CI: 1.025–1.511) than that in girls (OR: 1.055, 95%CI: 0.818–1.361). The association between NO2 exposure and risk of EDVs for asthma was stronger in school-age children [OR ranged from 1.376 (95%CI: 1.044–1.813) to 3.607 (95%CI: 1.552–8.385) across different lags] than that in preschool-age children, whereas the association between PM10 exposure and risk of EDVs for asthma was greater in preschool-age children [OR ranged from 1.873 (95%CI: 1.022–3.433) to 1.878 (95%CI: 1.028–3.431)] than that in school-age children. We observed an association of risk of EDVs for asthma with daytime air pollution exposure, but not with night-time air pollution exposure. This study suggests that risk of childhood asthma exacerbations increases within a few hours of air pollution exposure.