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The equilibrium constant for water dimerization (KP) was determined as a function of temperature via rigorous calculation of the canonical (H2O)2 partition function using the recently developed split Wigner pseudo-spectral method and the VRT(ASP-W) pair potential. Our KP(T) values are significantly larger than those from previous theoretical treatments but somewhat smaller than literature experimental values, which exist, however, only over a very limited temperature range. These results indicate that water dimers can exist in sufficient concentrations (e.g., 10¹⁶ cm^-3 at 40 °C and 100% relative humidity) to affect physical and chemical processes in the atmosphere.