Little is known about the acute effects of ozone on morbidity risk in China. We conducted a time-series study to examine the association between ozone and daily emergency room visits (ERVs) in Beijing, China. We identified 7,088,309 ERVs between January 1, 2014 and December 31, 2015. A generalized additive model with Poisson regression incorporating penalized spline functions was employed to analyze ERVs in association with daily 8-h maximum ozone concentrations. An increase of 10 μg/m3 of same-day ozone concentration was significantly associated with a 0.24% (95% CI, 0.21%-0.26%), 0.31% (95% CI, 0.27%-0.35%), and 0.43% (95% CI, 0.36%-0.50%) increase in daily ERVs for the whole study period, days when the daily 8-h maximum ozone met the Chinese Ambient Air Quality Standards (CAAQS) Grade II standard, and days that met the CAAQS Grade I standard, respectively. These results were robust when considering the potential confounding effects of PM2.5, PM10, NO2, SO2, and CO. In conclusion, our findings suggested significant effects of ozone exposure on daily ERVs in Beijing. Improving air quality with even lower ozone level than the current CAAQS could yield important public health.