The ozone (O3) concentrations in island cities are low due to the relatively low concentrations of O3 precursors. However, Pingtan, a typical island city along the southeast coast of China, suffers from frequent O3 pollution. In this study, one year of hourly O3 concentration data collected from three sites (rural, suburban, and urban) on Pingtan were used to investigate the characteristics and formation mechanism of O3 pollution. The results showed that the minimum O3 concentrations measured at the three sites were larger than 55.53 µg m–3, which was likely caused by low NO titration over Pingtan. The O3 concentrations in summer were low because of the East Asian monsoon. The daily maximum O3 concentrations with sea and land breezes (SLBs) were higher than those without SLBs at all three sites (8.52, 9.84, and 14.30 µg m–3 at the rural, suburban, and urban sites, respectively). In addition, SLBs amplified the diurnal variation in O3 concentrations. Cluster analysis and the analysis of an episode of high O3 concentration indicated that the developed Yangtze River Delta is the main source of O3 in Pingtan. This study helps reveal the characteristics and formation mechanism of O3 pollution in island cities.