A record-breaking severe ozone (O₃) pollution episode occurred in the Pearl River Delta (PRD) in early autumn 2019 when the PRD was under the influence of a Pacific subtropical high followed by Typhoon Mitag. In this study, we analyzed the effects of meteorological and photochemical processes on the O₃ concentration in the PRD during this episode by carrying out the Weather Research Forecast–Community Multiscale Air Quality (WRF-CMAQ) model simulations. Results showed that low relative humidity, high boundary layer height, weak northerly surface wind, and strong downdrafts were the main meteorological factors contributing to O₃ pollution. Moreover, delayed sea breezes that lasted into the night would transport O₃ from the sea back to the land and resulted in secondary O₃ maxima at night. In addition, O₃ and its precursors stored in the residual layer above the surface layer at night can be mixed down to the surface in the next morning, further enhancing the daytime ground-level O₃ concentration on the following day. Photochemical production of O₃, with a daytime average production rate of about 7.2 ppb h⁻¹ (parts per billion), is found to be the predominate positive contributor to the O₃ budget of the boundary layer (0–1260 m) during the entire O₃ episode, while the horizontal and vertical transport fluxes are the dominant negative contributors. This O₃ episode accounted for 10 out of the yearly total of 51 d when the maximum daily 8 h average (MDA8) O₃ concentration exceeded the national standard of 75 ppb in the PRD in 2019. Based on these results, we propose that the enhanced photochemical production of O₃ during the episode is a major cause of the most severe O₃ pollution year since the official O₃ observation started in the PRD in 2006. Moreover, since this O₃ episode is a synoptic-scale phenomenon covering the entire eastern China, we also suggest that the enhanced photochemical production of O₃ in this O₃ episode is a major cause of the extraordinarily high O₃ concentrations observed in eastern China in 2019.