Sea fog has an important impact on marine activities, coastal ecological environments, and regional radiation budgets. In the present study, we analyzed the characteristics of sea fog frequency (SFF) interdecadal variations over the northern South China Sea (NSCS) and the role of the Pacific Decadal Oscillation (PDO) in these changes, using independent datasets. The SFFs in the fog season derived from International Comprehensive Ocean–Atmosphere Data Set (ICOADS) observations and ERA-5 cross-validate a significant decadal transition from negative to positive phase in the late 1970s, which was further verified by a moving t-test. The composite fields after 1980s, including wind, low-atmospheric stability and moisture supply, were much more favorable for an increase in SFF than those before 1980s. These conditions were found to be closely associated with the PDO, with a significant correlation coefficient between the SFF derived from ICOADS (ERA-5) and the PDO at 0.53 (0.48), and even reaching 0.94 (0.92) after applying an 18-year moving average from 1950 to 1999. An anticyclonic circulation anomaly over the North western Pacific associated with positive PDO phase enhances the stability in the lower atmosphere and the water vapor supply due to a southerly wind anomaly at 850 hPa over the NSCS. With the strengthening of the winter monsoon in the positive PDO phase, northerly wind anomalies over the East Asian continent converge with southerly wind anomalies from the ocean, increasing the humidity in the NSCS, which helps to incease the SFF. In the negative PDO phase, the oceanic and atmospheric conditions slightly decrease the frequency of sea fog occurrence. This study improves the understanding of the mechanism on the interdecadal variation of SFF as modulated by the PDO, and has useful implications for further work on sea fog projection.