A hybrid quantum mechanical and molecular mechanical potential is used in Monte Carlo simulations to examine the solvent effects on the electronic excitation energy of the n → π* transition of acetone in ambient and supercritical water fluid, in which the temperature is in the range of 25-500°C with pressures of 1-2763 atm. In the present study, the acetone molecule is described by the AM1 Hamiltonian, and the water molecules are treated classically. Two sets of calculations are performed. The first involves the TIP4P model for water, and the second employs a polarizable model, POL2, for the solvent. The first calculation yields the excitation energy by using the static ground-state solvent charge distribution obtained from QM-CI/MM calculations. The latter takes into account the effect of solvent polarization following the solute electronic excitation. The trend of the computed n → π* blue-shifts for acetone as function of the fluid density is in good agreement with experimental results. The present simulations of acetone in the supercritical, near supercritical, dense-liquid, and ambient water fluids reveal that the solvatochromic shifts are dominated by the electrostatic interactions between acetone and water molecules during the solute excitation. Additionally, the solvent charge redistribution following the solute electronic excitation has a small correlation (0 to -37 cm-1) to the total solvatochromic shift and decreases linearly with water density. Both the solvatochromic shift and solvent polarization correction are more obvious in the ambient water than in the supercritical water because the solvent stabilization of the ground state over the excited state is more significant in the former condition.