We examine characteristics of the low-frequency variability of sea surface temperature (SST) in the East/Japan Sea (EJS) based on SST data longer than 100 years (1891-2005). The SST anomaly variability in the EJS has strong seasonal variations in terms of its spatial structure and amplitude and the total SST anomaly variability is largely explained by that during the boreal winter rather than that during the boreal summer. The EJS SST index, which is defined as the time series of the monthly SST anomaly averaged in the EJS basin, shows a primary spectral density at a frequency longer than a decade. The variability of low-frequency EJS SST is characterized by five distinct events of the temperature changes. The most striking feature is that the EJS SST experienced two significant warming events during the 20th century, i.e., from the early 1940s to the late 1940s, and from the mid 1980s to the present. In particular, the second period of warming since the mid 1980s is not limited to a certain season unlike the first period of warming. Our result indicates that the atmospheric variability associated with the EJS SST variability during winter exhibits significant manifestations over both the Siberian High pressure and the Aleutian Low pressure. Significant warming of EJS SST during winter could be associated with changes in the two pressure systems under different contributions.