Abstract: The sea surface pH in the Northwestern Pacific Ocean shows significant long-term and seasonal variation with an apparent acidification trend. In the past decade, the Kuroshio Extension Observatory (KEO) moored buoy in the Kuroshio extension (144°36′E, 32°24′N) has provided a wealth of data. Based on the field measured salinity, temperature (°C), atmospheric pressure (hPa), and CO₂ partial pressure (pCO₂, μatm) from the sea surface, the near-situ seawater surface pH data was reconstructed from 2008 to 2019 using the CO2SYS program. Then, its decadal trend and seasonal variation were analyzed. At the KEO site, the rate of sea surface pH decline from 2008 to 2019 was (−2.18±0.52)×10⁻³/a, consistent with the trend in the coastal sea around Japan. The climatological monthly average pH reached its minimum in summer, with seasonal variations similar to those observed at two other sites at the same latitude in the western North Pacific (137°E, 32°N and 165°E, 32°N). At the same time, the seasonal variation was more significant than the nearby coastal Central Laboratory site (located in Chiba Prefecture). The maximum rate of ocean acidification occurred in summer (−3.22±0.64)×10⁻³/a, which was comparable to autumn (−2.55±0.44)×10⁻³/a, followed by the spring (−1.89±0.42)×10⁻³/a, and the lowest rate occurred in winter (−1.66±0.37)×10⁻³/a, which was about twice less than the maximum. Considering the high uncertainties due to data gaps and the short length of the time series in this study (less than or equal to 12 years), more extended time series are suggested to be used to further investigate the decadal and seasonal trends in ocean acidification. This research could enrich our understanding of the accelerated ocean acidification rate in the Kuroshio extension area.