The excited-state proton-transfer (ESPT) reaction of 5-cyano-2-naphthol (5CN2) has been investigated in sub- and supercritical water using time-resolved fluorescence measurements. Under ambient conditions, a very fast decay of the fluorescence from the excited state of normal 5CN2 (ROH*) and a simultaneous increase of the fluorescence from the excited state of the anion species (RO-*) were observed, as reported previously. The very high ESPT rate was evaluated as 0.12 ps(-1). With increasing temperature at a constant pressure of 39.0 MPa, the proton transfer became slow. At 615 K and 39.0 MPa, another fluorescence from a new unknown chemical species appeared, which was assigned to the contact ion pair (CIP) of RO-* and the hydronium ion. With decreasing pressure at 664 K, the fluorescence from RO-* disappeared, and the fluorescence from ROH* and CIP was observed. At the very low density of supercritical water, only the fluorescence decay of ROH* was detected. The reaction dynamics was analyzed with the help of singular value decomposition and spectral decomposition using model functions. The ESPT rate was correlated with the solvent dielectric constant and/or the hydrogen-bonding ability.