Steady changes of temperature and oxygen concentration since the 1950s have been observed in the East Sea. In addition to temperature and oxygen, this study extends the analysis to salinity observations. Salinity has increased between 300 and 1000 m and decreased below 1500 m with a trend of about 0.06 and -0.02 psu/century, respectively. A simple diagnostic inverse model indicates that the changes in the intermediate depth (300--1000 m) were primarily due to a change in salinity at the surface outcrop, and those in the deep and bottom water were due to not only a warming of the surface source water but also a circulation change in the deep and bottom water, with similar magnitudes of contributions from the two factors. Both the observations and model consistently suggest that the water mass structures have changed during the last few decades.