The metal cluster size in supported metal catalysts impacts the oxidation state of the metal atoms, coordination capability, and finally the catalytic activity—especially when the number of atoms becomes countable. The correlation between metal oxidation state and its catalytic consequences for ultrafine Pt was studied for photocatalytic overall water splitting using a Pt/SrTiO3 (photo)catalyst. A distinctive change in catalytic behavior and oxidation state was observed below 100 Pt-atom clusters at ∼2 nm. Combining density functional theory (DFT) and experimental characterizations including X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), the smaller Pt clusters obtained by the surface organometallic route (under 100 Pt atoms) were predominantly oxidized and selectively performed photocatalytic water splitting selectively without activation of the water-forming back-reaction from H2 and O2. When the Pt clusters obtained by classical impregnation were larger than 2 nm, they remained metallic (Pt0) and were active for both water splitting and the competing thermal water formation back reaction. In addition, Pt0 clusters are poisoned in the presence of CO, whereas highly dispersed ultra-fine oxidized Pt clusters are insensitive to CO. This paper presents evidence of ultrafine PtOx (below approximately 2 nm clusters) that are insensitive to coordination of various gas identities (H2, O2, CO), resulting in efficient and selective photocatalytic overall water splitting.