Based on the air quality monitoring data collected in 40 cities in Yangtze River Delta urban agglomeration in 2016, this study presents the spatial-temporal variation of O₃ concentration in Yangtze River Delta urban agglomeration in 2016 via spatial interpolation, spatial autocorrelation analysis, and hotspot analysis. It shows that:1) with an average O₃ non-attainment rate of 8.8percent, O₃ has become the second major source of pollutants following PM_2.5; 2) the O₃ concentration is decreasing from summer, spring, autumn to winter, the monthly changing of O₃ concentration follows a pattern of "M", with two peaks in May and August respectively, and a valley in June due to the interruption of the plum rain season; 3) the O₃ non-attainment mainly occurs from April to September, which contributes 98.1% to the O₃ non-attainment days during the whole year with an average monthly rate of 17.3%; 4) the O₃ concentration shows an a general decrease trend from the northeast to southwest, and the line connecting Hangzhou and Ma'anshan highlights the difference between the highly polluted area in the eastern side of the line, and the less polluted area in the western side of the line. The cities around the Taihu Lake suffer from severest pollution. In geologically speaking, the spatial distribution of O₃ is approximately in accord with that of the economic development levels of Yangtze River Delta urban agglomeration; 5) the O₃ concentration follows the spatial agglomeration law. Owing to the impact of southeast monsoon, the hotspots of O₃ are primarily distributed in the eastern part of the Lake Cities from April to July, and later, move westward to Nanjing and its adjacent areas from August to September.