The Optimum Multiparameter (OMP) method was used to determine the relative contribution of water masses in the East China Sea (ECS) based on their measured temperature, salinity, and dissolved inorganic iodine species (DIIS, iodate and iodide) from recent oceanographic cruises in June 2014, 2015, and 2016. DIIS (as quasi‐conservative parameters) were added first into the OMP analysis, which enabled us to quantitatively estimate the water composition in the extremely dynamic and productive ECS. The OMP results indicate that seawater in the ECS mainly consisted of Changjiang Diluted Water (CDW), Taiwan Strait Warm Water (TSWW), Kuroshio Surface Water (KSW), and Kuroshio Subsurface Water (KSSW). In June 2014, 2015, and 2016, the KSSW dominated in the deep water, intruded into the ECS shelf from the northeast of Taiwan and bifurcated into a nearshore branch and offshore branch. The nearshore branch could reach north of 29°N off the Zhejiang coast. When comparing the KSSW intrusion in June 2014, 2015, and 2016, the average KSSW contribution of the bottom KSSW area in June 2016 (78.9%) was greater than in the previous 2 years (67.9% and 64.5%). In the northern transects (north of 28°N), the KSSW intrusion in June 2016 also contained the greatest average KSSW contribution and the maximum coverage area. Thus, the KSSW intrusion was strongest in June 2016 and may be comparable in June 2014 and 2015. This interannual variability of the KSSW intrusion is likely related to the El Niño‐Southern Oscillation (ENSO) rather than the Pacific Decadal Oscillation (PDO).