An ab initio three-body potential (called cc) is presented, discussed, and applied. Using the ab initio two-body potential of Matsuoka-Clementi-Yoshimine (1975) and the CC potential, we have carried out Metropolis-Monte Carlo simulation at 298 K for an (N, V, T) ensemble with N = 64, 125, and 343 water molecules. Improvements, relative to the use of only the two-body potential, have been obtained for the g(O–O), g(O–H), g(H–H), g(N) correlation functions, for the X-ray and neutron beam scattering intensities, and for the enthalpy.