A cobalt-phosphate (Co-P-i) oxygen evolution cocatalyst on a SrTiO3 photoelectrode was investigated by Co and P K-edge X-ray absorption fine structure (XAFS) spectroscopy. Formation of an edge-sharing CoO6 octahedral structure nanocluster in the Co-P-i cocatalyst was verified by the Co K-edge XAFS spectra under the photoelectrochemical reaction. The P K-edge XAFS spectrum for the Co-P-i cocatalyst showed a pre-edge peak derived from the hybridization of Co(3d)-O(2p)-P(3p) valence orbitals. Therefore, it was spectroscopically demonstrated that a phosphate species binds to the Co edge of the CoO6 cluster in the Co-P-i cocatalyst. The phosphate species is likely to stabilize the CoO6 cluster for efficient oxygen evolution activity.