We present a prominent quasi--decadal variability of fall extreme wave heights (H90, September--November mean of the monthly 90th percentile of significant wave heights) in the western North Pacific (WNP), based on wave reanalysis of the ERA40. It is found by applying an Empirical Orthogonal Function (EOF) analysis to H90 for 1957--2001 that the first EOF mode captures a monopole structure with maximum amplitude in the south of Japan with quasi--decadal variability of approximately 15--year. The quasi--decadal variability is also found partially in H90 observed around southern coast of Japan. By comparing intense tropical cyclone (ITC, with central pressure below 980 hPa) tracks for the decennial periods, we may be able to attribute the quasi--decadal variability of H90 in the WNP to the changes in the ITC tracks around the south of Japan.