China has implemented some air pollution management measures in recent years, yet severe ozone pollution remains a significant issue. The Southeastern Coast of China (SECC) is often influenced by hot extremes and tropical cyclones (TCs), and the two can occur simultaneously (TC-HDs). The compound TC-HDs show a rising trend in the summers of 2014–2019, potentially affecting ozone pollution. Here, we found that surface ozone concentrations over SECC are elevated during extreme hot days than the climatology. However, compared to extreme hot days alone (AHDs), the maximum 8-hour average ozone (MDA8 O₃) concentration increases by an average of 6.8 μg/m³ in the Pearl River Delta (PRD) and decreases by 13.2 μg/m³ in the Yangtze River Delta (YRD) during the compound TC-HDs. The meteorological conditions during AHDs favor the chemical production of ozone over SECC, exhibiting increased temperature and solar radiation but decreased relative humidity. Relative to AHDs, strong northeasterly winds prevail in SECC during TC-HDs, suggesting the potential of ozone cross-regional transport between YRD and PRD. The process analysis in the chemical transport model (GEOS-Chem) suggests that relative to AHDs, the chemical production of ozone is enhanced in YRD during TC-HDs while horizontal transport alleviates ozone pollution in YRD but worsens it in PRD through cross-regional transport. The results highlight the significant effects of cross-regional transport in modulating ozone pollution in the two megacity clusters during hot extremes accompanied by TC activities, giving insight into future ozone control measures over SECC under global warming.