Photocatalytic water splitting takes place at the semiconductor/electrolyte interface. Although the reactions are strongly affected by the subtle changes in the interface structure, little is known about the interface from an atomistic point of view. In this study, we investigate the GaN(0001)/water interface structure by combining first-principles calculation and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). In particular, the relationship between the geometric and electronic structure of the interface is revealed. First, the evolution of the GaN/water interface structure upon water adsorption is predicted from first-principles calculations. Computational results indicate that (1) at low coverage (below 3/4 monolayer), the Fermi level is pinned to the surface states originating from surface Ga atom dangling bonds, and water adsorbs dissociatively, forming oxygen atoms as well as hydroxyl groups, and (2) at higher coverage (above 3/4 monolayer), the Fermi level becomes free from the pinning, and adsorption of intact water becomes dominant. AP-XPS measurements were carried out for the water coverage ranging from submonolayer (low coverage) to several monolayers (high coverage). The core-level binding energies calculated from first-principles were used successfully to assign the adsorbate species to experimental O 1s peaks. Both the electronic and geometric structures predicted by the first-principles calculation explain well the experimental spectra obtained by the AP-XPS measurements. The results demonstrate that the combined spectroscopic and first-principles computational approach offers a detailed atomic level understanding of the solid/liquid interface structures.