The reaction pathways for water dissociation at a model liquid-solid interface have been investigated by a combination of experimental and theoretical approaches. By scanning tunneling microscopy (STM) and high-resolution electron energy-loss spectroscopy (HREELS), we revealed that the fragments of condensed water molecules, i.e., OH and H, efficiently terminate the isolated dangling bonds on a precovered Si(001) surface, in comparison with those of the isolated water molecules on the same surface. The most favorable reaction mechanism was predicted by first-principles calculations. At the first stage, the condensed water molecules create a new surface OH group at one of the isolated dangling bond sites. Simultaneously, counter fragment H and surrounding water molecules form a flexible hydronium complex along hydrogen bonds, because the fragment H takes a certain positive charge. Then, another dangling bond is terminated by a H fragment under the proton relay mechanism via the hydronium complex, in which a very low activation energy is expected because the hydronium complex near the surface is not sufficiently stabilized as in the case of aqueous liquid but is hindered in shallow potential energy surfaces. Since the spatial hindrance near solid surfaces is a common property, the characteristic proton pathway should appear at various aqueous liquid-solid interfaces and enhance the surface reactions involving proton transfer.