Abstract: The effects of ENSO on the interannual variability of winter fog days over southern China and the interdecadal variation of the relationship between them are examined using the EOF analysis and other statistical methods based on the observed daily fog data from 52 meteorological stations in southern China and ERA5 reanalysis data from 1979 to 2016. The results are shown as follows: (1) The winter fog days over southern China (WFDSC) exhibit a whole-region consistent pattern and display evident interannual variability and a significant increasing trend. The WFDSC is closely associated with a weaker northerly wind, a more stable atmosphere, and a decreasing T−T_d near the surface, which implies that the water vapor in air is increasing, providing favorable meteorological background fields for the preservation and growth of fog over southern China. (2) ENSO has a close association with the interannual variability of WFDSC, which is mostly determined by heavy fog days rather than light fog days. During El Niño winters, an anomalous anticyclone forms over the Philippine Sea, and the warm moist anomaly southwesterlies on the western flank of this anticyclone brings abundant water vapor to southern China, facilitating fog formation. During La Niña winters, there are almost opposite changes of fog days. (3) The interannual ENSO-WFDSC relationship shows a clear interdecadal change around 1996, with a low correlation during the period of 1979–1996 but a significant correlation after 1997. Further analysis reveals that this change in the relationship may be modulated by the Atlantic Multidecadal Oscillation (AMO).