Abstract: Extreme humid-heat events directly affect human health and socioeconomic development. In China, such events mainly occur in the southeast region in summer. This study revealed a weak relationship between early winter El Niño and Southern Oscillation (ENSO) and summer extreme humid-heat days (EHHDs) in southeastern China during 1961–1993. A significant positive correlation was observed between ENSO and EHHDs during 1994–2020. The increase in EHHDs caused by El Niño events was the main reason for the strong correlation. In the latter period, when El Niño events occurred in the early winter, the western North Pacific anomalous anticyclone (WNPAC) was westerly and stronger in summer. It caused a sinking motion, downward shortwave radiation, and water vapor transport anomalies, which were conducive to increasing the number of EHHDs. Further analysis indicated that during the latter period, cold sea surface temperature (SST) anomalies in the equatorial central–eastern Pacific and warm SST anomalies over the Maritime Continent in the summer following an El Niño event were more pronounced compared to those in the former period. The warm SST anomalies near the Maritime Continent enhanced local convection, leading to anomalous upward motion that diverged in the upper troposphere. On the one hand, this upward motion and the cold SST anomalies in the equatorial central–eastern Pacific in the zonal direction jointly suppressed convective activity and precipitation anomalies over the equatorial central–western Pacific. This suppression further excited a Gill-type response, reinforcing the WNPAC. On the other hand, the anomalous upward motion over the Maritime Continent strengthened the meridional Hadley circulation and contributed to intensifying the WNPAC. Additionally, during the latter period, a circumglobal teleconnection wave train was observed in the mid-high latitudes in the summer following an El Niño event. This wave train may have enhanced the WNPAC by inducing upward motion over Northeast Asia and modulating the meridional circulation. In contrast, during the former period, the summer SST anomalies and the mid-high-latitude circumglobal teleconnection were weaker, resulting in negligible impacts on the WNPAC and EHHDs.