The redox-active tyrosine YD (D2-Tyr160) in photosystem II (PSII) serves as a side-path electron donor to P680. When YD is oxidized, a proton is released from phenolic OH, and a neutral radical YD* is formed. A hydrogen bond network around YD must be deeply involved in the mechanism of the YD reaction. In this study, we have detected water molecules structurally coupled to YD by means of Fourier transform infrared (FTIR) spectroscopy. Light-induced YD*/YD FTIR difference spectrum of a hydrated film of the PSII core complexes from Thermosynechococcus elongatus showed major signals at 3636(-)/3617(+) and 3594(+)/3585(-) cm-1 in the weakly hydrogen bonded OH stretching region. These peaks downshifted by 11-12 cm-1 upon H218O substitution and almost disappeared upon H/D exchange, and hence, they were definitely assigned to the water OH vibrations. Small intramolecular couplings of 3-6 cm-1 estimated from the OH frequencies of residual HOD species in a deuterated film indicate that these OH signals arise from two different water molecules that have significantly asymmetric hydrogen bond structures. Similar OH signals were observed in PSII-enriched membranes from spinach, suggesting that two water molecules commonly exist near YD irrespective of biological species. These water molecules are coupled to YD most probably through a hydrogen bond network or one of them possibly interacts directly with YD, and thus, they may play crucial roles in the YD reaction by forming a proton-transfer pathway and tuning the redox potential of YD.